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From:   Michael Kelley <mikelley@microsoft.com>
To:     Long Li <longli@microsoft.com>,
        "longli@linuxonhyperv.com" <longli@linuxonhyperv.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
CC:     KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>
Subject: RE: [PATCH 2/2] Drivers: hv: add XStore Fastpath driver
Thread-Topic: [PATCH 2/2] Drivers: hv: add XStore Fastpath driver
Thread-Index: AQHXXAJLl0OGg22fU0mIFDqhwVn9YascP8mQgAXSAQCAAB+fIA==
Date:   Wed, 23 Jun 2021 22:01:45 +0000
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From: Long Li <longli@microsoft.com> Sent: Wednesday, June 23, 2021 12:21 P=
M
> >
> > From: longli@linuxonhyperv.com <longli@linuxonhyperv.com> Sent:
> > Monday, June 7, 2021 6:05 PM
> > >
> > > Add XStore Fastpath driver to support accelerated access to Azure Blo=
b
> > > Storage Services.
> > >
> > > Cc: K. Y. Srinivasan <kys@microsoft.com>
> > > Cc: Haiyang Zhang <haiyangz@microsoft.com>
> > > Cc: Stephen Hemminger <sthemmin@microsoft.com>
> > > Cc: Wei Liu <wei.liu@kernel.org>
> > > Cc: Dexuan Cui <decui@microsoft.com>
> > > Signed-off-by: Long Li <longli@microsoft.com>
> > > ---
> > >  MAINTAINERS                 |   1 +
> > >  drivers/hv/Kconfig          |  11 +
> > >  drivers/hv/Makefile         |   1 +
> > >  drivers/hv/channel_mgmt.c   |   6 +
> > >  drivers/hv/hv_xs_fastpath.c | 579
> > > ++++++++++++++++++++++++++++++++++++++++++++
> > >  drivers/hv/hv_xs_fastpath.h |  82 +++++++
> > >  include/linux/hyperv.h      |   9 +
> > >  7 files changed, 689 insertions(+)
> > >  create mode 100644 drivers/hv/hv_xs_fastpath.c  create mode 100644
> > > drivers/hv/hv_xs_fastpath.h
> > >
> > > diff --git a/MAINTAINERS b/MAINTAINERS index 9487061..b547eb9 100644
> > > --- a/MAINTAINERS
> > > +++ b/MAINTAINERS
> > > @@ -8440,6 +8440,7 @@ M:	Haiyang Zhang <haiyangz@microsoft.com>
> > >  M:	Stephen Hemminger <sthemmin@microsoft.com>
> > >  M:	Wei Liu <wei.liu@kernel.org>
> > >  M:	Dexuan Cui <decui@microsoft.com>
> > > +M:	Long Li <longli@microsoft.com>
> > >  L:	linux-hyperv@vger.kernel.org
> > >  S:	Supported
> > >  T:	git git://git.kernel.org/pub/scm/linux/kernel/git/hyperv/linux.gi=
t
> >
> > Seems like adding you to the Hyper-V maintainers list should be a separ=
ate
> > patch.  Having "maintainer" status is unrelated to adding this new driv=
er.
>=20
> There was a warning from checkpatch.pl. It asks me to add to maintainers =
in the same patch
> of the driver.
>=20

I've also seen this warning when adding a new file.  I think it is just
asking you to check if the new file is already covered by a maintainer.
For any files added to the drivers/hv directory, like here, we
already have maintainers specified, and the warning can be
ignored.  The logic in checkpatch.pl isn't sophisticated enough
to figure out for sure whether there are already maintainers
specified.

> >
> > > diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig index
> > > 66c794d..2128a8b 100644
> > > --- a/drivers/hv/Kconfig
> > > +++ b/drivers/hv/Kconfig
> > > @@ -27,4 +27,15 @@ config HYPERV_BALLOON
> > >  	help
> > >  	  Select this option to enable Hyper-V Balloon driver.
> > >
> > > +config HYPERV_XS_FASTPATH
> > > +	tristate "Microsoft Azure XStore Fastpath driver"
> > > +	depends on HYPERV && X86_64
> > > +	help
> > > +	  Select this option to enable Microsoft Azure XStore Fastpath driv=
er.
> > > +
> > > +	  This driver supports accelerated Microsoft Azure XStore Blob acce=
ss.
> > > +	  To compile this driver as a module, choose M here. The module wil=
l be
> > > +	  called hv_xs_fastpath.
> > > +
> > > +
> > >  endmenu
> > > diff --git a/drivers/hv/Makefile b/drivers/hv/Makefile index
> > > 94daf82..080fe88 100644
> > > --- a/drivers/hv/Makefile
> > > +++ b/drivers/hv/Makefile
> > > @@ -2,6 +2,7 @@
> > >  obj-$(CONFIG_HYPERV)		+=3D hv_vmbus.o
> > >  obj-$(CONFIG_HYPERV_UTILS)	+=3D hv_utils.o
> > >  obj-$(CONFIG_HYPERV_BALLOON)	+=3D hv_balloon.o
> > > +obj-$(CONFIG_HYPERV_XS_FASTPATH)+=3D hv_xs_fastpath.o
> > >
> > >  CFLAGS_hv_trace.o =3D -I$(src)
> > >  CFLAGS_hv_balloon.o =3D -I$(src)
> > > diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> > > index 6fd7ae5..a3f156e 100644
> > > --- a/drivers/hv/channel_mgmt.c
> > > +++ b/drivers/hv/channel_mgmt.c
> > > @@ -153,6 +153,12 @@
> > >  	  .allowed_in_isolated =3D false,
> > >  	},
> > >
> > > +	/* XStore Fastpath */
> > > +	{ .dev_type =3D HV_XS_FASTPATH,
> > > +	  HV_XS_FASTPATH_GUID,
> > > +	  .perf_device =3D true,
> >
> > I'm curious about setting .perf_device=3Dtrue.  The other perf devices =
are
> > those that support multiple VMbus channels, but this device has only a =
single
> > channel.  Is there another reason to set .perf_device=3Dtrue?
> >
> > Also set .allowed_in_isolated to "false" so that it is explicitly calle=
d out like
> > the other entries in the table.
>=20
> Will fix this.
>=20
> >
> > > +	},
> > > +
> > >  	/* Unknown GUID */
> > >  	{ .dev_type =3D HV_UNKNOWN,
> > >  	  .perf_device =3D false,
> > > diff --git a/drivers/hv/hv_xs_fastpath.c b/drivers/hv/hv_xs_fastpath.=
c
> > > new file mode 100644 index 0000000..ee4152e
> > > --- /dev/null
> > > +++ b/drivers/hv/hv_xs_fastpath.c
> > > @@ -0,0 +1,579 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> >
> > Let's have an offline discussion about the license choice here and in
> > hv_xs_fastpath.h.
> >
> > > +/*
> > > + * Copyright (c) 2021, Microsoft Corporation.
> > > + *
> > > + * Authors:
> > > + *   Long Li <longli@microsoft.com>
> > > + */
> > > +#include <linux/kernel.h>
> > > +#include <linux/module.h>
> > > +#include <linux/device.h>
> > > +#include <linux/slab.h>
> > > +#include <linux/cred.h>
> > > +#include <linux/debugfs.h>
> > > +#include <linux/pagemap.h>
> > > +#include "hv_xs_fastpath.h"
> > > +
> > > +#ifdef CONFIG_DEBUG_FS
> > > +struct dentry *xs_fastpath_debugfs_root; #endif
> > > +
> > > +static struct xs_fastpath_device xs_fastpath_dev;
> > > +
> > > +static int xs_fastpath_ringbuffer_size =3D (128 * 1024);
> > > +module_param(xs_fastpath_ringbuffer_size, int, 0444);
> > > +MODULE_PARM_DESC(xs_fastpath_ringbuffer_size, "Ring buffer size
> > > +(bytes)");
> > > +
> > > +static const struct hv_vmbus_device_id id_table[] =3D {
> > > +	{ HV_XS_FASTPATH_GUID,
> > > +	  .driver_data =3D 0
> > > +	},
> > > +	{ },
> > > +};
> > > +
> > > +#define XS_ERR 0
> > > +#define XS_WARN 1
> > > +#define XS_DBG 2
> > > +static int log_level =3D XS_WARN;
> > > +module_param(log_level, int, 0644);
> > > +MODULE_PARM_DESC(logging_level,
> >
> > s/logging_level/log_level/
> >
> > > +	"Log level, 0 - Error, 1 - Warning (default), 3 - Debug.");
> >
> > s/Log level, 0/Log level: 0/
> > s/3 - Debug/2 - Debug/
> >
> > > +
> > > +#define xs_fastpath_log(level, fmt, args...)	\
> > > +do {	\
> > > +	if (level <=3D log_level)	\
> > > +		pr_err("%s:%d " fmt, __func__, __LINE__, ##args);	\
> > > +} while (0)
> > > +
> > > +#define xs_fastpath_dbg(fmt, args...) xs_fastpath_log(XS_DBG, fmt,
> > > +##args) #define xs_fastpath_warn(fmt, args...)
> > > +xs_fastpath_log(XS_WARN, fmt, ##args) #define xs_fastpath_err(fmt,
> > > +args...) xs_fastpath_log(XS_ERR, fmt, ##args)
> > > +
> > > +struct xs_fastpath_vsp_request_ctx {
> > > +	struct list_head list;
> > > +	struct completion wait_vsp;
> > > +	struct xs_fastpath_request_sync *request; };
> >
> > Any reason for this structure definition to be here instead of in
> > hv_xs_fastpath.h?
>=20
> I will restructure the code and move most driver private data structures =
to .c file.
>=20
> >
> > > +
> > > +static void xs_fastpath_on_channel_callback(void *context) {
> > > +	struct vmbus_channel *channel =3D (struct vmbus_channel *)context;
> > > +	const struct vmpacket_descriptor *desc;
> > > +
> > > +	xs_fastpath_dbg("entering interrupt from vmbus\n");
> > > +	foreach_vmbus_pkt(desc, channel) {
> > > +		struct xs_fastpath_vsp_request_ctx *request_ctx;
> > > +		struct xs_fastpath_vsp_response *response;
> > > +		u64 cmd_rqst =3D vmbus_request_addr(&channel->requestor,
> > > +					desc->trans_id);
> > > +		if (cmd_rqst =3D=3D VMBUS_RQST_ERROR) {
> > > +			xs_fastpath_err("Incorrect transaction id %llu\n",
> > > +				desc->trans_id);
> > > +			continue;
> > > +		}
> > > +		request_ctx =3D (struct xs_fastpath_vsp_request_ctx *)cmd_rqst;
> > > +		response =3D hv_pkt_data(desc);
> > > +
> > > +		xs_fastpath_dbg("got response for request %pUb status %u"
> > > +			"response_len %u\n",
> > > +			&request_ctx->request->guid, response->error,
> > > +			response->response_len);
> > > +		request_ctx->request->response.status =3D response->error;
> > > +		request_ctx->request->response.response_len =3D
> > > +			response->response_len;
> > > +		complete(&request_ctx->wait_vsp);
> > > +	}
> >
> > This for loop could potentially go on for a long time if there were lot=
s of
> > responses in the ring buffer, or responses being added while this loop =
is
> > running.  It seems like there should be some timeout, and the remaining
> > work rescheduled to prevent it from running too long.  I know this code=
 is
> > just like the code in storvsc, but storvsc_on_channel_callback() has th=
e same
> > problem, which is on my list to fix.
>=20
> This is a good point. I don't think this is a problem for remote storage =
drivers, as they are on
> request-response protocol, and completions can't stall the CPU. Other sto=
rage drivers like
> NVMe is using a similar loop in nvme_process_cq().
>=20
> >
> > > +
> > > +}
> > > +
> > > +static int xs_fastpath_fop_open(struct inode *inode, struct file
> > > +*file) {
> > > +	atomic_inc(&xs_fastpath_dev.file_count);
> > > +	if (xs_fastpath_dev.removing) {
> > > +		if (atomic_dec_and_test(&xs_fastpath_dev.file_count))
> > > +			wake_up(&xs_fastpath_dev.wait_files);
> > > +		return -ENODEV;
> > > +	}
> > > +
> > > +	file->private_data =3D &xs_fastpath_dev;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int xs_fastpath_fop_release(struct inode *inode, struct file
> > > +*file) {
> > > +	if (atomic_dec_and_test(&xs_fastpath_dev.file_count))
> > > +		wake_up(&xs_fastpath_dev.wait_files);
> > > +	return 0;
> > > +}
> > > +
> > > +static inline bool xs_fastpath_safe_file_access(struct file *file) {
> > > +	return file->f_cred =3D=3D current_cred() && !uaccess_kernel(); }
> > > +
> > > +static int get_buffer_pages(int rw, void __user *buffer, u32 buffer_=
len,
> > > +	struct page ***ppages, size_t *start, size_t *num_pages) {
> > > +	struct iovec iov;
> > > +	struct iov_iter iter;
> > > +	int ret;
> > > +	ssize_t result;
> > > +	struct page **pages;
> > > +
> > > +	ret =3D import_single_range(rw, buffer, buffer_len, &iov, &iter);
> > > +	if (ret) {
> > > +		xs_fastpath_err("request buffer access error %d\n", ret);
> > > +		return ret;
> > > +	}
> > > +	xs_fastpath_dbg("iov_iter type %d offset %lu count %lu nr_segs %lu\=
n",
> > > +		iter.type, iter.iov_offset, iter.count, iter.nr_segs);
> > > +
> > > +	result =3D iov_iter_get_pages_alloc(&iter, &pages, buffer_len, star=
t);
> > > +	if (result < 0) {
> > > +		xs_fastpath_err("failed to ping user pages result=3D%ld\n", result=
);
> >
> > s/ping/pin/   [??]
> >
> > > +		return result;
> > > +	}
> > > +	if (result !=3D buffer_len) {
> > > +		xs_fastpath_err("can't ping user pages requested %d got %ld\n",
> >
> > s/ping/pin/   [??]
> >
> > > +			buffer_len, result);
> > > +		return -EFAULT;
> > > +	}
> > > +
> > > +	*ppages =3D pages;
> > > +	*num_pages =3D (result + *start + PAGE_SIZE - 1) / PAGE_SIZE;
> > > +	return 0;
> > > +}
> >
> > Can any of the xs_fastpath_err() calls be caused by a user error, such =
as
> > passing in bad values on an ioctl call?  If so, we probably want to not=
 print a
> > console message in those cases.
> >
> > > +
> > > +static void fill_in_page_buffer(u64 *pfn_array,
> > > +	int *index, struct page **pages, unsigned long num_pages) {
> > > +	int i, page_idx =3D *index;
> > > +
> > > +	for (i =3D 0; i < num_pages; i++)
> > > +		pfn_array[page_idx++] =3D page_to_pfn(pages[i]);
> > > +	*index =3D page_idx;
> > > +}
> >
> > See notes below.  Filling in the pfn_array needs to account for running=
 on an
> > ARM64 system where PAGE_SIZE !=3D HV_HYP_PAGE_SIZE.
>=20
> The driver is currently conditioned on X86_64 in kconfig, as there is no =
host driver and no
> way to test it on ARM64. I suggest adding support to ARM64 if needed in f=
uture.

I'm not completely in agreement with the idea of solving the issues with
different PAGE_SIZE later, but as you say, the driver is conditioned on
X86_64 in Kconfig, so I won't argue the point.

>=20
> >
> > > +
> > > +static void free_buffer_pages(size_t num_pages, struct page **pages)
> > > +{
> > > +	unsigned long i;
> > > +
> > > +	for (i =3D 0; i < num_pages; i++)
> > > +		if (pages[i])
> > > +			put_page(pages[i]);
> > > +	kfree(pages);
> >
> > Doesn't this need to be kvfree()?  'pages' is allocated with kvmalloc()=
, which
> > could return vmalloc memory.
>=20
> Will fix this.
>=20
> >
> > > +}
> > > +
> > > +static long xs_fastpath_ioctl_user_request(struct file *filp,
> > > +unsigned long arg) {
> > > +	struct xs_fastpath_device *dev =3D filp->private_data;
> > > +	char __user *argp =3D (char __user *) arg;
> > > +	struct xs_fastpath_request_sync request;
> > > +	struct xs_fastpath_vsp_request_ctx *request_ctx;
> > > +	unsigned long flags;
> > > +	int ret;
> > > +	size_t request_start, request_num_pages =3D 0;
> > > +	size_t response_start, response_num_pages =3D 0;
> > > +	size_t data_start, data_num_pages =3D 0, total_num_pages;
> > > +	struct page **request_pages =3D NULL, **response_pages =3D NULL;
> > > +	struct page **data_pages =3D NULL;
> > > +	struct vmbus_packet_mpb_array *desc;
> > > +	u64 *pfn_array;
> > > +	int desc_size;
> > > +	int page_idx;
> > > +	struct xs_fastpath_vsp_request *vsp_request;
> > > +
> > > +	if (dev->removing)
> > > +		return -ENODEV;
> >
> > I think this test is just to avoid starting a new request if we know th=
at the
> > device has been marked for removing.  But because there's no
> > synchronization, it's possible for the "removing" flag to be set immedi=
ately
> > after the test has passed.
>=20
> The usage of this flag "removing" here is to provide a fast failure path =
if a user-mode keeps
> requesting while the device is being removed, thus saving CPU cycles if a=
 malicious user-
> mode is doing this. It's okay to remove the check of this flag and it doe=
sn't affect the
> correctness of the device removal logic. I prefer to leave the check as i=
s.
>=20
> >
> > > +
> > > +	if (!xs_fastpath_safe_file_access(filp)) {
> > > +		xs_fastpath_err("process %d(%s) changed security contexts after"
> > > +			" opening file descriptor\n",
> > > +			task_tgid_vnr(current), current->comm);
> >
> > I don't think we should produce a console message, at least not a "err"
> > level.  It gives a user the ability to generate an arbitrary number of =
console
> > messages.  Maybe "dbg" level would be OK.
> >
> > > +		return -EACCES;
> > > +	}
> > > +
> > > +	if (!access_ok(argp, sizeof(struct xs_fastpath_request_sync))) {
> > > +
> > > +		xs_fastpath_err("don't have permission to user provide buffer\n");
> > > +		return -EFAULT;
> > > +	}
> >
> > Is this check needed at all?  Again, we don't want to generate a consol=
e
> > message due to a user error, and copy_from_user() below
> > will validate that the access is OK.   A "dbg" level message would be
> > OK.
>=20
> Yes, this can be removed as copy_from_user() will guarantee permission is=
 correct.
>=20
> >
> > > +
> > > +	if (copy_from_user(&request, argp, sizeof(request)))
> > > +		return -EFAULT;
> > > +
> > > +	xs_fastpath_dbg("xs_fastpath ioctl request guid %pUb timeout %u req=
uest_len %u"
> > > +		" response_len %u data_len %u request_buffer %llx "
> > > +		"response_buffer %llx data_buffer %llx\n",
> > > +		&request.guid, request.timeout, request.request_len,
> > > +		request.response_len, request.data_len, request.request_buffer,
> > > +		request.response_buffer, request.data_buffer);
> > > +
> > > +	if (!request.request_len || !request.response_len)
> > > +		return -EINVAL;
> > > +
> > > +	request_ctx =3D kzalloc(sizeof(*request_ctx), GFP_KERNEL);
> > > +	if (!request_ctx)
> > > +		return -ENOMEM;
> >
> > Could the request_ctx just be on the stack?  Below, it stores a pointer=
 to the
> > request, which is on the stack, and the request is accessed on the
> > onchannel_callback function.
> > So having request_ctx on the stack wouldn't be the only usage of a stac=
k
> > variable in the onchannel_callback function.
>=20
> I can move to request_ctx to stack.
>=20
> >
> > > +
> > > +	init_completion(&request_ctx->wait_vsp);
> > > +	request_ctx->request =3D &request;
> > > +
> > > +	/*
> > > +	 * Need to set rw to READ to have page table set up for passing to =
VSP.
> > > +	 * Setting it to WRITE will cause the page table entry not allocate=
d
> > > +	 * as it's waiting on Copy-On-Write on next page fault. This doesn'=
t
> > > +	 * work in this scenario because VSP wants all the pages to be pres=
ent.
> > > +	 */
> > > +	ret =3D get_buffer_pages(READ, (void __user *)request.request_buffe=
r,
> > > +		request.request_len, &request_pages, &request_start,
> > > +		&request_num_pages);
> > > +	if (ret)
> > > +		goto get_user_page_failed;
> > > +
> > > +	ret =3D get_buffer_pages(READ, (void __user *)request.response_buff=
er,
> > > +		request.response_len, &response_pages, &response_start,
> > > +		&response_num_pages);
> > > +	if (ret)
> > > +		goto get_user_page_failed;
> > > +
> > > +	if (request.data_len) {
> > > +		ret =3D get_buffer_pages(READ,
> > > +			(void __user *) request.data_buffer, request.data_len,
> > > +			&data_pages, &data_start, &data_num_pages);
> > > +		if (ret)
> > > +			goto get_user_page_failed;
> > > +	}
> >
> > Do the request_len, response_len, and data_len need to be validated
> > before being passed to get_buffer_pages() since they are passed in from
> > user space?  The fields are 32 bits, so the max value is 4 Gbytes, whic=
h maybe
> > is OK.  But with XS_FASTPATH_MAX_PAGES set to 8192, the maximum size is
> > 32 Mbytes with a 4K page size, and 512 Mbytes with a 64K page size.
>=20
> The VSP support a maximum of 32MB of buffer pages combined, so
> XS_FASTPATH_MAX_PAGES is set to 8192. The check is done to make
> sure we don't pass excessive pages to VSP. I'll check on request_len,
> response_len and data_len before allocating pages.
>=20

Got it.  But since the VSP limit is based on size (32 MB), not pages, could
the check be done based on size rather than page count?  This would be
one less thing to fix later to handle ARM64 page sizes.

> >
> > > +
> > > +	total_num_pages =3D request_num_pages + response_num_pages +
> > > +				data_num_pages;
> > > +	if (total_num_pages > XS_FASTPATH_MAX_PAGES) {
> >
> > Note that this check imposes different limits depending on PAGE_SIZE.
> > Is the value XS_FASTPATH_MAX_PAGES governed by the largest
> > vmbus_packet_mpb_array that Hyper-V will accept?  If not, does there ne=
ed
> > to be a check against the max vmbus_packet_mpb_array size?
> > (I don't know what the Hyper-V limit is.)
> >
> > > +		xs_fastpath_err("number of DMA pages %lu buffer exceeding %u\n",
> > > +			total_num_pages, XS_FASTPATH_MAX_PAGES);
> > > +		ret =3D -EINVAL;
> > > +		goto get_user_page_failed;
> > > +	}
> > > +
> > > +	/* Construct a VMBUS packet and send it over to VSP */
> > > +	desc_size =3D sizeof(struct vmbus_packet_mpb_array) +
> > > +			sizeof(u64) * total_num_pages;
> >
> > The above doesn't handle the case where the guest PAGE_SIZE is differen=
t
> > from the Hyper-V page size (HV_HYP_PAGE_SIZE), as can happen on ARM64.
> >
> > > +	desc =3D kzalloc(desc_size, GFP_KERNEL);
> > > +	vsp_request =3D kzalloc(sizeof(*vsp_request), GFP_KERNEL);
> > > +	if (!desc || !vsp_request) {
> > > +		kfree(desc);
> > > +		kfree(vsp_request);
> > > +		ret =3D -ENOMEM;
> > > +		goto get_user_page_failed;
> > > +	}
> > > +
> > > +	desc->range.offset =3D 0;
> > > +	desc->range.len =3D total_num_pages * PAGE_SIZE;
> > > +	pfn_array =3D desc->range.pfn_array;
> > > +	page_idx =3D 0;
> > > +
> > > +	if (request.data_len) {
> > > +		fill_in_page_buffer(pfn_array, &page_idx, data_pages,
> > > +			data_num_pages);
> > > +		vsp_request->data_buffer_offset =3D data_start;
> > > +		vsp_request->data_buffer_length =3D request.data_len;
> > > +		vsp_request->data_buffer_valid =3D 1;
> > > +	}
> > > +
> > > +	fill_in_page_buffer(pfn_array, &page_idx, request_pages,
> > > +		request_num_pages);
> > > +	vsp_request->request_buffer_offset =3D request_start +
> > > +						data_num_pages * PAGE_SIZE;
> > > +	vsp_request->request_buffer_length =3D request.request_len;
> > > +
> > > +	fill_in_page_buffer(pfn_array, &page_idx, response_pages,
> > > +		response_num_pages);
> > > +	vsp_request->response_buffer_offset =3D response_start +
> > > +		(data_num_pages + request_num_pages) * PAGE_SIZE;
> > > +	vsp_request->response_buffer_length =3D request.response_len;
> >
> > All of the above that is filling in the pfn_array needs to handle the c=
ase
> > where PAGE_SIZE !=3D HV_HYP_PAGE_SIZE.
> >
> > > +
> > > +	vsp_request->version =3D 0;
> > > +	vsp_request->timeout_ms =3D request.timeout;
> >
> > Does the request.timeout value from user space need to be validated?
> > I'm thinking about a value of 0, or a really small value like 1 ms, or =
a really
> > large value that effectively means no timeout.
>=20
> The timeout is not used by the VSC/VSP kernel mode, so we don't mess up w=
ith it.

In that case, would it be better to just pass zero as the timeout in the
VSP request?

>=20
> >
> > > +	vsp_request->operation_type =3D XS_FASTPATH_DRIVER_USER_REQUEST;
> > > +	guid_copy(&vsp_request->transaction_id, &request.guid);
> > > +
> > > +	spin_lock_irqsave(&dev->vsp_pending_lock, flags);
> > > +	list_add_tail(&request_ctx->list, &dev->vsp_pending_list);
> > > +	spin_unlock_irqrestore(&dev->vsp_pending_lock, flags);
> > > +
> > > +	xs_fastpath_dbg("sending request to VSP\n");
> > > +	xs_fastpath_dbg("desc_size %u desc->range.len %u desc-range.offset =
%u\n",
> > > +		desc_size, desc->range.len, desc->range.offset);
> > > +	xs_fastpath_dbg("vsp_request data_buffer_offset %u data_buffer_leng=
th %u "
> > > +		"data_buffer_valid %u request_buffer_offset %u "
> > > +		"request_buffer_length %u response_buffer_offset %u "
> > > +		"response_buffer_length %u\n",
> > > +		vsp_request->data_buffer_offset,
> > > +		vsp_request->data_buffer_length,
> > > +		vsp_request->data_buffer_valid,
> > > +		vsp_request->request_buffer_offset,
> > > +		vsp_request->request_buffer_length,
> > > +		vsp_request->response_buffer_offset,
> > > +		vsp_request->response_buffer_length);
> > > +
> > > +	ret =3D vmbus_sendpacket_mpb_desc(dev->device->channel, desc, desc_=
size,
> > > +		vsp_request, sizeof(*vsp_request), (u64) request_ctx);
> > > +
> > > +	kfree(desc);
> > > +	kfree(vsp_request);
> > > +	if (ret)
> > > +		goto vmbus_send_failed;
> > > +
> > > +	wait_for_completion(&request_ctx->wait_vsp);
> >
> > This is a "wait forever", which exists in other places in the Hyper-V d=
rivers,
> > but we've learned that might not be a good idea.  It's more complicated=
 to
> > deal with a timeout, and the potential for the Hyper-V response to come=
 in
> > after the timeout, but that would be the most robust approach.  Maybe w=
e
> > can live with the "wait forever" approach for now, but in the long run =
we'll
> > probably want something like a 30 second or 60 second timeout.
> >
> > > +
> > > +	/*
> > > +	 * At this point, the response is already written to request
> > > +	 * by VMBUS completion handler, copy them to user-mode buffers
> > > +	 * and return to user-mode
> > > +	 */
> > > +	if (copy_to_user(argp +
> > > +			offsetof(struct xs_fastpath_request_sync,
> > > +				response.status),
> > > +			&request.response.status,
> > > +			sizeof(request.response.status))) {
> > > +		ret =3D -EFAULT;
> > > +		goto vmbus_send_failed;
> > > +	}
> > > +
> > > +	if (copy_to_user(argp +
> > > +			offsetof(struct xs_fastpath_request_sync,
> > > +				response.response_len),
> > > +			&request.response.response_len,
> > > +			sizeof(request.response.response_len)))
> > > +		ret =3D -EFAULT;
> > > +
> > > +vmbus_send_failed:
> > > +	spin_lock_irqsave(&dev->vsp_pending_lock, flags);
> > > +	list_del(&request_ctx->list);
> > > +	if (list_empty(&dev->vsp_pending_list))
> > > +		wake_up(&dev->wait_vsp);
> > > +	spin_unlock_irqrestore(&dev->vsp_pending_lock, flags);
> > > +
> > > +get_user_page_failed:
> > > +	free_buffer_pages(request_num_pages, request_pages);
> > > +	free_buffer_pages(response_num_pages, response_pages);
> > > +	free_buffer_pages(data_num_pages, data_pages);
> > > +
> > > +	kfree(request_ctx);
> > > +	return ret;
> > > +}
> > > +
> > > +static long xs_fastpath_fop_ioctl(struct file *filp, unsigned int cm=
d,
> > > +	unsigned long arg)
> > > +{
> > > +	long ret =3D -EIO;
> > > +
> > > +	switch (cmd) {
> > > +	case IOCTL_XS_FASTPATH_DRIVER_USER_REQUEST:
> > > +		if (_IOC_SIZE(cmd) !=3D sizeof(struct xs_fastpath_request_sync))
> > > +			return -EINVAL;
> > > +		ret =3D xs_fastpath_ioctl_user_request(filp, arg);
> > > +		break;
> > > +
> > > +	default:
> > > +		xs_fastpath_err("unrecognized IOCTL code %u\n", cmd);
> >
> > Again, probably don't want to output a console message just because use=
r
> > space passed a bad ioctl code.
> >
> > > +	}
> > > +
> > > +	return ret;
> > > +}
> > > +
> > > +static const struct file_operations xs_fastpath_client_fops =3D {
> > > +	.owner	=3D THIS_MODULE,
> > > +	.open	=3D xs_fastpath_fop_open,
> > > +	.unlocked_ioctl =3D xs_fastpath_fop_ioctl,
> > > +	.release =3D xs_fastpath_fop_release,
> > > +};
> > > +
> > > +static struct miscdevice xs_fastpath_misc_device =3D {
> > > +	MISC_DYNAMIC_MINOR,
> > > +	"azure_xs_fastpath",
> > > +	&xs_fastpath_client_fops,
> > > +};
> > > +
> > > +static int xs_fastpath_show_pending_requests(struct seq_file *m, voi=
d
> > > +*v) {
> > > +	unsigned long flags;
> > > +	struct xs_fastpath_vsp_request_ctx *request_ctx;
> > > +
> > > +	seq_puts(m, "List of pending requests\n");
> > > +	seq_puts(m, "UUID request_len response_len data_len "
> > > +		"request_buffer response_buffer data_buffer\n");
> > > +	spin_lock_irqsave(&xs_fastpath_dev.vsp_pending_lock, flags);
> > > +	list_for_each_entry(request_ctx, &xs_fastpath_dev.vsp_pending_list,=
 list) {
> > > +		seq_printf(m, "%pUb ", &request_ctx->request->guid);
> > > +		seq_printf(m, "%u ", request_ctx->request->request_len);
> > > +		seq_printf(m, "%u ", request_ctx->request->response_len);
> > > +		seq_printf(m, "%u ", request_ctx->request->data_len);
> > > +		seq_printf(m, "%llx ", request_ctx->request-request_buffer);
> > > +		seq_printf(m, "%llx ", request_ctx->request-response_buffer);
> > > +		seq_printf(m, "%llx\n", request_ctx->request->data_buffer);
> > > +	}
> > > +	spin_unlock_irqrestore(&xs_fastpath_dev.vsp_pending_lock, flags);
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int xs_fastpath_debugfs_open(struct inode *inode, struct file
> > > +*file) {
> > > +	return single_open(file, xs_fastpath_show_pending_requests, NULL); =
}
> > > +
> > > +static const struct file_operations xs_fastpath_debugfs_fops =3D {
> > > +	.open		=3D xs_fastpath_debugfs_open,
> > > +	.read		=3D seq_read,
> > > +	.llseek		=3D seq_lseek,
> > > +	.release	=3D seq_release
> > > +};
> > > +
> > > +static void xs_fastpath_remove_device(struct xs_fastpath_device *dev=
)
> > > +{
> > > +	wait_event(dev->wait_files, atomic_read(&dev->file_count) =3D=3D 0)=
;
> >
> > After this wait_event() completes, but before misc_deregister() complet=
es,
> > the device can still be found and opened.  Because the removing flag is=
 set,
> > xs_fastpath_fop_open() won't do anything but increment the file_count,
> > then decrement it again, and call wake_up().  That all works with
> > xs_fastpath_dev as a static variable.  But it might not work if xs_fast=
path_dev
> > was dynamically allocated, as would be the case if multiple such device=
s were
> > supported.  So I'm wondering if this code really should do the
> > misc_deregister() and debugfs_remove_recursive() first, and then to the
> > wait_event().  When done in that order, you know that the device can't =
be
> > found again after the wait_event() completes.  Then the "removing" flag=
 is
> > not even needed.
>=20
> I don't think it's safe to call misc_deregister() while there are files o=
pened and I/O ongoing
> on the device. If you see we have this guarantee from misc_deregister() t=
o not mess up
> with ongoing I/O, please point me to the code.

But your code doesn't provide this safety because immediately after wait_ev=
ent()
returns due to the file_count being 0, another process could open the /dev
device and have it open at least for a short duration of time while it is
checking the removing flag.  So misc_deregister() could be happening in
parallel with the /dev device being open (though not with I/Os being
in progress).

I don't think there is any way for this driver (or any driver) to provide t=
he
kind of safety you describe, which is why I think misc_deregister() must ha=
ndle
it, though I haven't gone and looked specifically for the code that does so=
.=20

>=20
> I don't see the need for supporting multiple xs_fastpath_dev. It is dynam=
ically allocated on
> vmbus probe, currently there can be only one such device on the system.

OK

>=20
> >
> > > +	misc_deregister(&xs_fastpath_misc_device);
> > > +#ifdef CONFIG_DEBUG_FS
> > > +	debugfs_remove_recursive(xs_fastpath_debugfs_root);
> > > +#endif
> > > +	/* At this point, we won't get any requests from user-mode */ }
> > > +
> > > +static int xs_fastpath_create_device(struct xs_fastpath_device *dev)
> > > +{
> > > +	int rc;
> > > +	struct dentry *d;
> > > +
> > > +	atomic_set(&xs_fastpath_dev.file_count, 0);
> > > +	init_waitqueue_head(&xs_fastpath_dev.wait_files);
> > > +	rc =3D misc_register(&xs_fastpath_misc_device);
> > > +	if (rc) {
> > > +		xs_fastpath_err("misc_register failed rc %d\n", rc);
> > > +		return rc;
> > > +	}
> > > +
> > > +#ifdef CONFIG_DEBUG_FS
> > > +	xs_fastpath_debugfs_root =3D debugfs_create_dir("xs_fastpath", NULL=
);
> > > +	if (!IS_ERR_OR_NULL(xs_fastpath_debugfs_root)) {
> > > +		d =3D debugfs_create_file("pending_requests", 0400,
> > > +			xs_fastpath_debugfs_root, NULL,
> > > +			&xs_fastpath_debugfs_fops);
> > > +		if (IS_ERR_OR_NULL(d)) {
> > > +			xs_fastpath_err("failed to create debugfs file\n");
> > > +			debugfs_remove_recursive(xs_fastpath_debugfs_root);
> > > +			xs_fastpath_debugfs_root =3D NULL;
> > > +		}
> > > +	} else
> > > +		xs_fastpath_err("failed to create debugfs root\n"); #endif
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static int xs_fastpath_connect_to_vsp(struct hv_device *device, u32
> > > +ring_size) {
> > > +	int ret;
> > > +
> > > +	spin_lock_init(&xs_fastpath_dev.vsp_pending_lock);
> > > +	INIT_LIST_HEAD(&xs_fastpath_dev.vsp_pending_list);
> > > +	init_waitqueue_head(&xs_fastpath_dev.wait_vsp);
> > > +	xs_fastpath_dev.removing =3D false;
> > > +
> > > +	xs_fastpath_dev.device =3D device;
> > > +	device->channel->rqstor_size =3D xs_fastpath_ringbuffer_size /
> > > +		sizeof(struct xs_fastpath_vsp_request);
> >
> > That setting probably isn't correct.  Presumably the Hyper-V VSP can ho=
ld a
> > certain number of requests that it has already removed the
> > guest->host ring buffer.  So the max number of in-flight requests
> > would be that Hyper-V VSP count, plus the number of requests that will =
fit in
> > the ring buffer.
> >
> > Independent of this setting, the ioctl's are synchronous, so the total =
number
> > of in-flight requests will be the number of threads
> > making requests.   Note that these ioctl calls include all blobs
> > being accessed using this method by all users running in the VM.
> > Nonetheless, it might be reasonable to just put a fixed cap on that num=
ber.
> > Perhaps something like 1024?  If the limit is exceeded, the calls to
> > vmbus_sendpacket_mpb_desc() will start failing because of being unable =
to
> > allocate a requestID.
>=20
> The VSC is designed to support asynchronous I/O requests, so this number =
can be
> potentially large. The reason why I put the largest possible number for r=
qstor_size (based
> on ring buffer) is not having it limit other queue depths in the Fastpath=
 code path.

I don't understand why the rqstor_size would be based on the ring buffer
size.  rqstor_size needs to accommodate all in-flight requests, including t=
hose
that the VSP may have already removed from the ring buffer but which are
not yet complete.

>=20
> >
> > Presumably the memory management subsystem will also limit the amount
> > of memory being pinned by the ioctl calls, which could also cause the i=
octl
> > calls to return an error.
> >
> >
> > > +
> > > +	ret =3D vmbus_open(device->channel, ring_size, ring_size, NULL, 0,
> > > +			xs_fastpath_on_channel_callback, device->channel);
> > > +
> > > +	if (ret) {
> > > +		xs_fastpath_err("failed to connect to VSP ret %d\n", ret);
> > > +		return ret;
> > > +	}
> > > +
> > > +	hv_set_drvdata(device, &xs_fastpath_dev);
> > > +
> > > +	return ret;
> > > +}
> > > +
> > > +static void xs_fastpath_remove_vmbus(struct hv_device *device) {
> > > +	struct xs_fastpath_device *dev =3D hv_get_drvdata(device);
> > > +	unsigned long flags;
> > > +
> > > +	spin_lock_irqsave(&dev->vsp_pending_lock, flags);
> > > +	if (!list_empty(&dev->vsp_pending_list)) {
> >
> > I don't think this can ever happen.  If the device has already been rem=
oved
> > by xs_fastpath_remove_device(), then we know that the device isn't open
> > in any processes, so there can't be any ioctl's in progress that would =
have
> > entries in the vsp_pending_list.
>=20
> The VSC is designed to support asynchronous I/O (while not implemented in=
 this version),
> so vsp_pending_list is needed if a user-mode decides to close the file an=
d not waiting for
> I/O.
>=20
> >
> > > +		spin_unlock_irqrestore(&dev->vsp_pending_lock, flags);
> > > +		xs_fastpath_dbg("wait for vsp_pending_list\n");
> > > +		wait_event(dev->wait_vsp,
> > > +			list_empty(&dev->vsp_pending_list));
> > > +	} else
> > > +		spin_unlock_irqrestore(&dev->vsp_pending_lock, flags);
> > > +
> > > +	/* At this point, no VSC/VSP traffic is possible over vmbus */
> > > +	hv_set_drvdata(device, NULL);
> > > +	vmbus_close(device->channel);
> > > +}
> > > +
> > > +static int xs_fastpath_probe(struct hv_device *device,
> > > +			const struct hv_vmbus_device_id *dev_id) {
> > > +	int rc;
> > > +
> > > +	xs_fastpath_dbg("probing device\n");
> > > +
> > > +	rc =3D xs_fastpath_connect_to_vsp(device, xs_fastpath_ringbuffer_si=
ze);
> > > +	if (rc) {
> > > +		xs_fastpath_err("error connecting to VSP rc %d\n", rc);
> > > +		return rc;
> > > +	}
> > > +
> > > +	// create user-mode client library facing device
> > > +	rc =3D xs_fastpath_create_device(&xs_fastpath_dev);
> > > +	if (rc) {
> > > +		xs_fastpath_remove_vmbus(device);
> > > +		return rc;
> > > +	}
> > > +
> > > +	xs_fastpath_dbg("successfully probed device\n");
> > > +	return 0;
> > > +}
> > > +
> > > +static int xs_fastpath_remove(struct hv_device *dev) {
> > > +	struct xs_fastpath_device *device =3D hv_get_drvdata(dev);
> > > +
> > > +	device->removing =3D true;
> > > +	xs_fastpath_remove_device(device);
> > > +	xs_fastpath_remove_vmbus(dev);
> > > +	return 0;
> > > +}
> > > +
> > > +static struct hv_driver xs_fastpath_drv =3D {
> > > +	.name =3D KBUILD_MODNAME,
> > > +	.id_table =3D id_table,
> > > +	.probe =3D xs_fastpath_probe,
> > > +	.remove =3D xs_fastpath_remove,
> > > +	.driver =3D {
> > > +		.probe_type =3D PROBE_PREFER_ASYNCHRONOUS,
> > > +	},
> > > +};
> > > +
> > > +static int __init xs_fastpath_drv_init(void) {
> > > +	int ret;
> > > +
> > > +	ret =3D vmbus_driver_register(&xs_fastpath_drv);
> > > +	return ret;
> > > +}
> > > +
> > > +static void __exit xs_fastpath_drv_exit(void) {
> > > +	vmbus_driver_unregister(&xs_fastpath_drv);
> > > +}
> > > +
> > > +MODULE_LICENSE("GPL");
> >
> > Let's discuss the license choice offline.
> >
> > > +MODULE_DESCRIPTION("Microsoft Azure XStore Storage Fastpath
> > driver");
> >
> > The word "Storage" seems unnecessary.  Everywhere else the name is just
> > "Azure Xstore Fastpath".
> >
> > > +module_init(xs_fastpath_drv_init);
> > > +module_exit(xs_fastpath_drv_exit);
> > > diff --git a/drivers/hv/hv_xs_fastpath.h b/drivers/hv/hv_xs_fastpath.=
h
> > > new file mode 100644 index 0000000..6db1904
> > > --- /dev/null
> > > +++ b/drivers/hv/hv_xs_fastpath.h
> > > @@ -0,0 +1,82 @@
> > > +/* SPDX-License-Identifier: GPL-2.0-only */
> > > +/*
> > > + * Copyright (c) 2021, Microsoft Corporation.
> > > + *
> > > + * Authors:
> > > + *   Long Li <longli@microsoft.com>
> > > + */
> > > +#ifndef _XS_FASTPATH_H
> > > +#define _XS_FASTPATH_H
> > > +
> > > +#include <linux/hyperv.h>
> > > +#include <linux/miscdevice.h>
> > > +#include <linux/hashtable.h>
> > > +#include <linux/uio.h>
> > > +
> > > +struct xs_fastpath_device {
> > > +	struct hv_device *device;
> > > +	bool removing;
> > > +
> > > +	struct list_head vsp_pending_list;
> > > +	spinlock_t vsp_pending_lock;
> > > +	wait_queue_head_t wait_vsp;
> > > +
> > > +	wait_queue_head_t wait_files;
> > > +	atomic_t file_count;
> > > +};
> > > +
> > > +/* user-mode sync request sent through ioctl */ struct
> > > +xs_fastpath_request_sync_response {
> > > +	__u32 status;
> > > +	__u32 response_len;
> > > +};
> > > +
> > > +struct xs_fastpath_request_sync {
> > > +	guid_t guid;
> > > +	__u32 timeout;
> > > +	__u32 request_len;
> > > +	__u32 response_len;
> > > +	__u32 data_len;
> > > +	__aligned_u64 request_buffer;
> > > +	__aligned_u64 response_buffer;
> > > +	__aligned_u64 data_buffer;
> > > +	struct xs_fastpath_request_sync_response response; };
> >
> > Wouldn't the structures used by user space need to go in a uapi header =
file?
>=20
> I will move those to uapi.
>=20
> >
> > > +
> > > +/* VSP messages */
> > > +enum xs_fastpath_vsp_request_type {
> > > +	XS_FASTPATH_DRIVER_REQUEST_FIRST     =3D 0x100,
> > > +	XS_FASTPATH_DRIVER_USER_REQUEST      =3D 0x100,
> > > +	XS_FASTPATH_DRIVER_REGISTER_BUFFER   =3D 0x101,
> > > +	XS_FASTPATH_DRIVER_DEREGISTER_BUFFER =3D 0x102,
> > > +	XS_FASTPATH_DRIVER_REQUEST_MAX       =3D 0x103
> >
> > Most of these don't seem to be used in the code.  And it seems a bit
> > unexpected for the MAX value to not be the same as the last value
> > (DEREGISTER_BUFFER).
>=20
> Some are not used in this version of the driver; they are placed here for=
 protocol
> completeness.
>=20
> >
> > > +};
> > > +
> > > +/* VSC->VSP request */
> > > +struct xs_fastpath_vsp_request {
> > > +	u32 version;
> > > +	u32 timeout_ms;
> > > +	u32 data_buffer_offset;
> > > +	u32 data_buffer_length;
> > > +	u32 data_buffer_valid;
> > > +	u32 operation_type;
> > > +	u32 request_buffer_offset;
> > > +	u32 request_buffer_length;
> > > +	u32 response_buffer_offset;
> > > +	u32 response_buffer_length;
> > > +	guid_t transaction_id;
> > > +} __packed;
> > > +
> > > +/* VSP->VSC response */
> > > +struct xs_fastpath_vsp_response {
> > > +	u32 length;
> > > +	u32 error;
> > > +	u32 response_len;
> > > +} __packed;
> > > +
> > > +#define IOCTL_XS_FASTPATH_DRIVER_USER_REQUEST \
> > > +		_IOWR('R', 10, struct xs_fastpath_request_sync)
> >
> > How was the ioctl code decided?  Using 'R' and '10' seems to be in the =
range
> > assigned to /dev/random, per the file Documentation/userspace-
> > api/ioctl/ioctl-number.rst.
> >
> > Does this also need to go in a uapi header file?
>=20
> Will fix this.
>=20
> >
> > > +
> > > +#define XS_FASTPATH_MAX_PAGES 8192
> >
> > How was this value determined?  Would be good to leave a comment, and
> > the comment should speak to how the limit is handled when PAGE_SIZE
> > varies on different architectures.
> >
> > > +
> > > +#endif /* define _XS_FASTPATH_H */
> > > diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h index
> > > d1e59db..a1730c4 100644
> > > --- a/include/linux/hyperv.h
> > > +++ b/include/linux/hyperv.h
> > > @@ -772,6 +772,7 @@ enum vmbus_device_type {
> > >  	HV_FCOPY,
> > >  	HV_BACKUP,
> > >  	HV_DM,
> > > +	HV_XS_FASTPATH,
> > >  	HV_UNKNOWN,
> > >  };
> > >
> > > @@ -1350,6 +1351,14 @@ int vmbus_allocate_mmio(struct resource
> > **new,
> > > struct hv_device *device_obj,
> > >  			  0x72, 0xe2, 0xff, 0xb1, 0xdc, 0x7f)
> > >
> > >  /*
> > > + * XStore Fastpath GUID
> > > + * {0590b792-db64-45cc-81db-b8d70c577c9e}
> > > + */
> > > +#define HV_XS_FASTPATH_GUID \
> > > +	.guid =3D GUID_INIT(0x0590b792, 0xdb64, 0x45cc, 0x81, 0xdb, \
> > > +			  0xb8, 0xd7, 0x0c, 0x57, 0x7c, 0x9e)
> > > +
> > > +/*
> > >   * Shutdown GUID
> > >   * {0e0b6031-5213-4934-818b-38d90ced39db}
> > >   */
> > > --
> > > 1.8.3.1
>=20
> I will address of rest of the comments.