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Date:   Thu, 13 Sep 2018 08:19:32 -0400
From:   Luiz Capitulino <lcapitulino@redhat.com>
To:     Pankaj Gupta <pagupta@redhat.com>
Cc:     kwolf@redhat.com, jack@suse.cz,
        xiaoguangrong eric <xiaoguangrong.eric@gmail.com>,
        kvm@vger.kernel.org, riel@surriel.com, linux-nvdimm@ml01.01.org,
        david@redhat.com, ross zwisler <ross.zwisler@intel.com>,
        linux-kernel@vger.kernel.org, qemu-devel@nongnu.org,
        hch@infradead.org, imammedo@redhat.com, mst@redhat.com,
        stefanha@redhat.com, niteshnarayanlal@hotmail.com,
        pbonzini@redhat.com, dan j williams <dan.j.williams@intel.com>,
        nilal@redhat.com
Subject: Re: [Qemu-devel] [PATCH 3/3]  virtio-pmem: Add virtio pmem driver
Message-ID: <20180913081932.14783262@doriath>
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        <20180831133019.27579-4-pagupta@redhat.com>
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On Thu, 13 Sep 2018 02:58:21 -0400 (EDT)
Pankaj Gupta <pagupta@redhat.com> wrote:

> Hi Luiz,
> 
> Thanks for the review.
> 
> >   
> > > This patch adds virtio-pmem driver for KVM guest.
> > > 
> > > Guest reads the persistent memory range information from
> > > Qemu over VIRTIO and registers it on nvdimm_bus. It also
> > > creates a nd_region object with the persistent memory
> > > range information so that existing 'nvdimm/pmem' driver
> > > can reserve this into system memory map. This way
> > > 'virtio-pmem' driver uses existing functionality of pmem
> > > driver to register persistent memory compatible for DAX
> > > capable filesystems.
> > > 
> > > This also provides function to perform guest flush over
> > > VIRTIO from 'pmem' driver when userspace performs flush
> > > on DAX memory range.
> > > 
> > > Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
> > > ---
> > >  drivers/virtio/Kconfig           |   9 ++
> > >  drivers/virtio/Makefile          |   1 +
> > >  drivers/virtio/virtio_pmem.c     | 255
> > >  +++++++++++++++++++++++++++++++++++++++
> > >  include/uapi/linux/virtio_ids.h  |   1 +
> > >  include/uapi/linux/virtio_pmem.h |  40 ++++++
> > >  5 files changed, 306 insertions(+)
> > >  create mode 100644 drivers/virtio/virtio_pmem.c
> > >  create mode 100644 include/uapi/linux/virtio_pmem.h
> > > 
> > > diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> > > index 3589764..a331e23 100644
> > > --- a/drivers/virtio/Kconfig
> > > +++ b/drivers/virtio/Kconfig
> > > @@ -42,6 +42,15 @@ config VIRTIO_PCI_LEGACY
> > >  
> > >  	  If unsure, say Y.
> > >  
> > > +config VIRTIO_PMEM
> > > +	tristate "Support for virtio pmem driver"
> > > +	depends on VIRTIO
> > > +	help
> > > +	This driver provides support for virtio based flushing interface
> > > +	for persistent memory range.
> > > +
> > > +	If unsure, say M.
> > > +
> > >  config VIRTIO_BALLOON
> > >  	tristate "Virtio balloon driver"
> > >  	depends on VIRTIO
> > > diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile
> > > index 3a2b5c5..cbe91c6 100644
> > > --- a/drivers/virtio/Makefile
> > > +++ b/drivers/virtio/Makefile
> > > @@ -6,3 +6,4 @@ virtio_pci-y := virtio_pci_modern.o virtio_pci_common.o
> > >  virtio_pci-$(CONFIG_VIRTIO_PCI_LEGACY) += virtio_pci_legacy.o
> > >  obj-$(CONFIG_VIRTIO_BALLOON) += virtio_balloon.o
> > >  obj-$(CONFIG_VIRTIO_INPUT) += virtio_input.o
> > > +obj-$(CONFIG_VIRTIO_PMEM) += virtio_pmem.o
> > > diff --git a/drivers/virtio/virtio_pmem.c b/drivers/virtio/virtio_pmem.c
> > > new file mode 100644
> > > index 0000000..c22cc87
> > > --- /dev/null
> > > +++ b/drivers/virtio/virtio_pmem.c
> > > @@ -0,0 +1,255 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/*
> > > + * virtio_pmem.c: Virtio pmem Driver
> > > + *
> > > + * Discovers persistent memory range information
> > > + * from host and provides a virtio based flushing
> > > + * interface.
> > > + */
> > > +#include <linux/virtio.h>
> > > +#include <linux/module.h>
> > > +#include <linux/virtio_ids.h>
> > > +#include <linux/virtio_config.h>
> > > +#include <uapi/linux/virtio_pmem.h>
> > > +#include <linux/spinlock.h>
> > > +#include <linux/libnvdimm.h>
> > > +#include <linux/nd.h>
> > > +
> > > +struct virtio_pmem_request {
> > > +	/* Host return status corresponding to flush request */
> > > +	int ret;
> > > +
> > > +	/* command name*/
> > > +	char name[16];
> > > +
> > > +	/* Wait queue to process deferred work after ack from host */
> > > +	wait_queue_head_t host_acked;
> > > +	bool done;
> > > +
> > > +	/* Wait queue to process deferred work after virt queue buffer avail */
> > > +	wait_queue_head_t wq_buf;
> > > +	bool wq_buf_avail;
> > > +	struct list_head list;
> > > +};
> > > +
> > > +struct virtio_pmem {
> > > +	struct virtio_device *vdev;
> > > +
> > > +	/* Virtio pmem request queue */
> > > +	struct virtqueue *req_vq;
> > > +
> > > +	/* nvdimm bus registers virtio pmem device */
> > > +	struct nvdimm_bus *nvdimm_bus;
> > > +	struct nvdimm_bus_descriptor nd_desc;
> > > +
> > > +	/* List to store deferred work if virtqueue is full */
> > > +	struct list_head req_list;
> > > +
> > > +	/* Synchronize virtqueue data */
> > > +	spinlock_t pmem_lock;
> > > +
> > > +	/* Memory region information */
> > > +	uint64_t start;
> > > +	uint64_t size;
> > > +};
> > > +
> > > +static struct virtio_device_id id_table[] = {
> > > +	{ VIRTIO_ID_PMEM, VIRTIO_DEV_ANY_ID },
> > > +	{ 0 },
> > > +};
> > > +
> > > + /* The interrupt handler */
> > > +static void host_ack(struct virtqueue *vq)
> > > +{
> > > +	unsigned int len;
> > > +	unsigned long flags;
> > > +	struct virtio_pmem_request *req, *req_buf;
> > > +	struct virtio_pmem *vpmem = vq->vdev->priv;
> > > +
> > > +	spin_lock_irqsave(&vpmem->pmem_lock, flags);
> > > +	while ((req = virtqueue_get_buf(vq, &len)) != NULL) {
> > > +		req->done = true;
> > > +		wake_up(&req->host_acked);
> > > +
> > > +		if (!list_empty(&vpmem->req_list)) {
> > > +			req_buf = list_first_entry(&vpmem->req_list,
> > > +					struct virtio_pmem_request, list);
> > > +			list_del(&vpmem->req_list);
> > > +			req_buf->wq_buf_avail = true;
> > > +			wake_up(&req_buf->wq_buf);
> > > +		}
> > > +	}
> > > +	spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> > > +}
> > > + /* Initialize virt queue */
> > > +static int init_vq(struct virtio_pmem *vpmem)
> > > +{
> > > +	struct virtqueue *vq;
> > > +
> > > +	/* single vq */
> > > +	vpmem->req_vq = vq = virtio_find_single_vq(vpmem->vdev,
> > > +				host_ack, "flush_queue");
> > > +	if (IS_ERR(vq))
> > > +		return PTR_ERR(vq);
> > > +
> > > +	spin_lock_init(&vpmem->pmem_lock);
> > > +	INIT_LIST_HEAD(&vpmem->req_list);
> > > +
> > > +	return 0;
> > > +};
> > > +
> > > + /* The request submission function */
> > > +static int virtio_pmem_flush(struct nd_region *nd_region)
> > > +{
> > > +	int err;
> > > +	unsigned long flags;
> > > +	struct scatterlist *sgs[2], sg, ret;
> > > +	struct virtio_device *vdev =
> > > +		dev_to_virtio(nd_region->dev.parent->parent);
> > > +	struct virtio_pmem *vpmem = vdev->priv;  
> > 
> > I'm missing a might_sleep() call in this function.  
> 
> I am not sure if we need might_sleep here? 
> We can add it as debugging aid for detecting any problems
> in sleeping from acquired atomic context?

Yes. Since this function sleeps and since some functions that
may run in atomic context call it, it's a good idea to
call might_sleep().

> > > +	struct virtio_pmem_request *req = kmalloc(sizeof(*req), GFP_KERNEL);
> > > +
> > > +	if (!req)
> > > +		return -ENOMEM;
> > > +
> > > +	req->done = req->wq_buf_avail = false;
> > > +	strcpy(req->name, "FLUSH");
> > > +	init_waitqueue_head(&req->host_acked);
> > > +	init_waitqueue_head(&req->wq_buf);
> > > +
> > > +	spin_lock_irqsave(&vpmem->pmem_lock, flags);
> > > +	sg_init_one(&sg, req->name, strlen(req->name));
> > > +	sgs[0] = &sg;
> > > +	sg_init_one(&ret, &req->ret, sizeof(req->ret));
> > > +	sgs[1] = &ret;  
> > 
> > It seems that sg_init_one() is only setting fields, in this
> > case you can move spin_lock_irqsave() here.  
> 
> yes, will move spin_lock_irqsave here.
> 
> >   
> > > +	err = virtqueue_add_sgs(vpmem->req_vq, sgs, 1, 1, req, GFP_ATOMIC);
> > > +	if (err) {
> > > +		dev_err(&vdev->dev, "failed to send command to virtio pmem device\n");
> > > +
> > > +		list_add_tail(&vpmem->req_list, &req->list);
> > > +		spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> > > +
> > > +		/* When host has read buffer, this completes via host_ack */
> > > +		wait_event(req->wq_buf, req->wq_buf_avail);
> > > +		spin_lock_irqsave(&vpmem->pmem_lock, flags);  
> > 
> > Is this error handling code assuming that at some point
> > virtqueue_add_sgs() will succeed for a different thread? If yes,
> > what happens if the assumption is false? That is, what happens if
> > virtqueue_add_sgs() never succeeds anymore?  
> 
> virtqueue_add_sgs will not succeed and corresponding thread should wait.
> All subsequent calling threads should also wait. As soon as there is first
> available free entry(from host), first waiting thread is acknowledged.
> 
> In worst case if Qemu is not utilizing any of the used buffer will keep
> multiple threads waiting. 
> 
> > 
> > Why not just return an error?  
> 
> As per suggestion by Stefan in previous discussion: if the virtqueue is full.  
> Printing a message and failing the flush isn't appropriate.  This thread needs to 
> wait until virtqueue space becomes available.

If virtqueue_add_sgs() is guaranteed to succeed at some point then OK.
Otherwise, you'll get threads getting stuck forever.

> > > +	}
> > > +	virtqueue_kick(vpmem->req_vq);
> > > +	spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> > > +
> > > +	/* When host has read buffer, this completes via host_ack */
> > > +	wait_event(req->host_acked, req->done);
> > > +	err = req->ret;  
> > 
> > If I'm understanding the QEMU code correctly, you're returning EIO
> > from QEMU if fsync() fails. I think this is wrong, since we don't know
> > if EIO in QEMU will be the same EIO in the guest. One way to solve this
> > would be to return 0 for success and 1 for failure from QEMU, and let the
> > guest implementation pick its error code (for your implementation it
> > could be EIO).  
> 
> Makes sense, will change this. 
> 
> Thanks,
> Pankaj 
> >   
> > > +	kfree(req);
> > > +
> > > +	return err;
> > > +};
> > > +EXPORT_SYMBOL_GPL(virtio_pmem_flush);
> > > +
> > > +static int virtio_pmem_probe(struct virtio_device *vdev)
> > > +{
> > > +	int err = 0;
> > > +	struct resource res;
> > > +	struct virtio_pmem *vpmem;
> > > +	struct nvdimm_bus *nvdimm_bus;
> > > +	struct nd_region_desc ndr_desc;
> > > +	int nid = dev_to_node(&vdev->dev);
> > > +	struct nd_region *nd_region;
> > > +
> > > +	if (!vdev->config->get) {
> > > +		dev_err(&vdev->dev, "%s failure: config disabled\n",
> > > +			__func__);
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	vdev->priv = vpmem = devm_kzalloc(&vdev->dev, sizeof(*vpmem),
> > > +			GFP_KERNEL);
> > > +	if (!vpmem) {
> > > +		err = -ENOMEM;
> > > +		goto out_err;
> > > +	}
> > > +
> > > +	vpmem->vdev = vdev;
> > > +	err = init_vq(vpmem);
> > > +	if (err)
> > > +		goto out_err;
> > > +
> > > +	virtio_cread(vpmem->vdev, struct virtio_pmem_config,
> > > +			start, &vpmem->start);
> > > +	virtio_cread(vpmem->vdev, struct virtio_pmem_config,
> > > +			size, &vpmem->size);
> > > +
> > > +	res.start = vpmem->start;
> > > +	res.end   = vpmem->start + vpmem->size-1;
> > > +	vpmem->nd_desc.provider_name = "virtio-pmem";
> > > +	vpmem->nd_desc.module = THIS_MODULE;
> > > +
> > > +	vpmem->nvdimm_bus = nvdimm_bus = nvdimm_bus_register(&vdev->dev,
> > > +						&vpmem->nd_desc);
> > > +	if (!nvdimm_bus)
> > > +		goto out_vq;
> > > +
> > > +	dev_set_drvdata(&vdev->dev, nvdimm_bus);
> > > +	memset(&ndr_desc, 0, sizeof(ndr_desc));
> > > +
> > > +	ndr_desc.res = &res;
> > > +	ndr_desc.numa_node = nid;
> > > +	ndr_desc.flush = virtio_pmem_flush;
> > > +	set_bit(ND_REGION_PAGEMAP, &ndr_desc.flags);
> > > +	nd_region = nvdimm_pmem_region_create(nvdimm_bus, &ndr_desc);
> > > +
> > > +	if (!nd_region)
> > > +		goto out_nd;
> > > +
> > > +	//virtio_device_ready(vdev);
> > > +	return 0;
> > > +out_nd:
> > > +	err = -ENXIO;
> > > +	nvdimm_bus_unregister(nvdimm_bus);
> > > +out_vq:
> > > +	vdev->config->del_vqs(vdev);
> > > +out_err:
> > > +	dev_err(&vdev->dev, "failed to register virtio pmem memory\n");
> > > +	return err;
> > > +}
> > > +
> > > +static void virtio_pmem_remove(struct virtio_device *vdev)
> > > +{
> > > +	struct virtio_pmem *vpmem = vdev->priv;
> > > +	struct nvdimm_bus *nvdimm_bus = dev_get_drvdata(&vdev->dev);
> > > +
> > > +	nvdimm_bus_unregister(nvdimm_bus);
> > > +	vdev->config->del_vqs(vdev);
> > > +	kfree(vpmem);
> > > +}
> > > +
> > > +#ifdef CONFIG_PM_SLEEP
> > > +static int virtio_pmem_freeze(struct virtio_device *vdev)
> > > +{
> > > +	/* todo: handle freeze function */
> > > +	return -EPERM;
> > > +}
> > > +
> > > +static int virtio_pmem_restore(struct virtio_device *vdev)
> > > +{
> > > +	/* todo: handle restore function */
> > > +	return -EPERM;
> > > +}
> > > +#endif
> > > +
> > > +
> > > +static struct virtio_driver virtio_pmem_driver = {
> > > +	.driver.name		= KBUILD_MODNAME,
> > > +	.driver.owner		= THIS_MODULE,
> > > +	.id_table		= id_table,
> > > +	.probe			= virtio_pmem_probe,
> > > +	.remove			= virtio_pmem_remove,
> > > +#ifdef CONFIG_PM_SLEEP
> > > +	.freeze                 = virtio_pmem_freeze,
> > > +	.restore                = virtio_pmem_restore,
> > > +#endif
> > > +};
> > > +
> > > +module_virtio_driver(virtio_pmem_driver);
> > > +MODULE_DEVICE_TABLE(virtio, id_table);
> > > +MODULE_DESCRIPTION("Virtio pmem driver");
> > > +MODULE_LICENSE("GPL");
> > > diff --git a/include/uapi/linux/virtio_ids.h
> > > b/include/uapi/linux/virtio_ids.h
> > > index 6d5c3b2..3463895 100644
> > > --- a/include/uapi/linux/virtio_ids.h
> > > +++ b/include/uapi/linux/virtio_ids.h
> > > @@ -43,5 +43,6 @@
> > >  #define VIRTIO_ID_INPUT        18 /* virtio input */
> > >  #define VIRTIO_ID_VSOCK        19 /* virtio vsock transport */
> > >  #define VIRTIO_ID_CRYPTO       20 /* virtio crypto */
> > > +#define VIRTIO_ID_PMEM         25 /* virtio pmem */
> > >  
> > >  #endif /* _LINUX_VIRTIO_IDS_H */
> > > diff --git a/include/uapi/linux/virtio_pmem.h
> > > b/include/uapi/linux/virtio_pmem.h
> > > new file mode 100644
> > > index 0000000..c7c22a5
> > > --- /dev/null
> > > +++ b/include/uapi/linux/virtio_pmem.h
> > > @@ -0,0 +1,40 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +/*
> > > + * This header, excluding the #ifdef __KERNEL__ part, is BSD licensed so
> > > + * anyone can use the definitions to implement compatible drivers/servers:
> > > + *
> > > + *
> > > + * Redistribution and use in source and binary forms, with or without
> > > + * modification, are permitted provided that the following conditions
> > > + * are met:
> > > + * 1. Redistributions of source code must retain the above copyright
> > > + *    notice, this list of conditions and the following disclaimer.
> > > + * 2. Redistributions in binary form must reproduce the above copyright
> > > + *    notice, this list of conditions and the following disclaimer in the
> > > + *    documentation and/or other materials provided with the distribution.
> > > + * 3. Neither the name of IBM nor the names of its contributors
> > > + *    may be used to endorse or promote products derived from this
> > > software
> > > + *    without specific prior written permission.
> > > + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
> > > ``AS IS''
> > > + * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
> > > THE
> > > + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
> > > PURPOSE
> > > + * ARE DISCLAIMED.  IN NO EVENT SHALL IBM OR CONTRIBUTORS BE LIABLE
> > > + * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
> > > CONSEQUENTIAL
> > > + * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
> > > + * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
> > > + * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
> > > STRICT
> > > + * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
> > > WAY
> > > + * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> > > + * SUCH DAMAGE.
> > > + *
> > > + * Copyright (C) Red Hat, Inc., 2018-2019
> > > + * Copyright (C) Pankaj Gupta <pagupta@redhat.com>, 2018
> > > + */
> > > +#ifndef _UAPI_LINUX_VIRTIO_PMEM_H
> > > +#define _UAPI_LINUX_VIRTIO_PMEM_H
> > > +
> > > +struct virtio_pmem_config {
> > > +	__le64 start;
> > > +	__le64 size;
> > > +};
> > > +#endif  
> > 
> > 
> >   
>