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Date:   Wed, 15 Sep 2021 03:30:53 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Wu Zongyong <wuzongyong@linux.alibaba.com>
Cc:     jasowang@redhat.com, virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, wei.yang1@linux.alibaba.com
Subject: Re: [PATCH v2 4/5] vdpa: add new vdpa attribute
 VDPA_ATTR_DEV_F_VERSION_1
Message-ID: <20210915032732-mutt-send-email-mst@kernel.org>
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On Wed, Sep 15, 2021 at 11:24:53AM +0800, Wu Zongyong wrote:
> On Tue, Sep 14, 2021 at 08:58:28AM -0400, Michael S. Tsirkin wrote:
> > On Tue, Sep 14, 2021 at 08:24:51PM +0800, Wu Zongyong wrote:
> > > This new attribute advertises whether the vdpa device is legacy or not.
> > > Users can pick right virtqueue size if the vdpa device is legacy which
> > > doesn't support to change virtqueue size.
> > > 
> > > Signed-off-by: Wu Zongyong <wuzongyong@linux.alibaba.com>
> > 
> > So if we are bothering with legacy, I think there are
> > several things to do when building the interface
> > - support transitional devices, that is allow userspace
> >   to tell device it's in legacy mode
> > - support reporting/setting supporting endian-ness
> 
> It's true if we try to implement a general drvier for legacy.
> But this series is dedicated to implement a driver only for ENI. Is it
> necessary to implement what you said here in this series?

To a certain degree, yes.

I am thinking about the UAPI here. The new attribute is part of that.
E.g. userspace consuming this needs to be more or less hardware agnostic
and not depend on specifics of ENI.

Otherwise if userspace assumes legacy==eni then it will break with
other hardware.
 
One way to test how generic it all is would be adding legacy support in
the simulator.

> > 
> > > ---
> > >  drivers/vdpa/vdpa.c          | 6 ++++++
> > >  drivers/virtio/virtio_vdpa.c | 7 ++++++-
> > >  include/uapi/linux/vdpa.h    | 1 +
> > >  3 files changed, 13 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
> > > index 1dc121a07a93..533d7f589eee 100644
> > > --- a/drivers/vdpa/vdpa.c
> > > +++ b/drivers/vdpa/vdpa.c
> > > @@ -12,6 +12,7 @@
> > >  #include <linux/slab.h>
> > >  #include <linux/vdpa.h>
> > >  #include <uapi/linux/vdpa.h>
> > > +#include <uapi/linux/virtio_config.h>
> > >  #include <net/genetlink.h>
> > >  #include <linux/mod_devicetable.h>
> > >  
> > > @@ -494,6 +495,7 @@ vdpa_dev_fill(struct vdpa_device *vdev, struct sk_buff *msg, u32 portid, u32 seq
> > >  	u16 max_vq_size;
> > >  	u32 device_id;
> > >  	u32 vendor_id;
> > > +	u64 features;
> > >  	void *hdr;
> > >  	int err;
> > >  
> > > @@ -508,6 +510,7 @@ vdpa_dev_fill(struct vdpa_device *vdev, struct sk_buff *msg, u32 portid, u32 seq
> > >  	device_id = vdev->config->get_device_id(vdev);
> > >  	vendor_id = vdev->config->get_vendor_id(vdev);
> > >  	max_vq_size = vdev->config->get_vq_num_max(vdev);
> > > +	features = vdev->config->get_features(vdev);
> > >  
> > >  	err = -EMSGSIZE;
> > >  	if (nla_put_string(msg, VDPA_ATTR_DEV_NAME, dev_name(&vdev->dev)))
> > > @@ -520,6 +523,9 @@ vdpa_dev_fill(struct vdpa_device *vdev, struct sk_buff *msg, u32 portid, u32 seq
> > >  		goto msg_err;
> > >  	if (nla_put_u16(msg, VDPA_ATTR_DEV_MAX_VQ_SIZE, max_vq_size))
> > >  		goto msg_err;
> > > +	if (features & BIT_ULL(VIRTIO_F_VERSION_1) &&
> > > +	    nla_put_flag(msg, VDPA_ATTR_DEV_VERSION_1))
> > > +		goto msg_err;
> > >  
> > >  	genlmsg_end(msg, hdr);
> > >  	return 0;
> > > diff --git a/drivers/virtio/virtio_vdpa.c b/drivers/virtio/virtio_vdpa.c
> > > index 72eaef2caeb1..1cba957c4cdc 100644
> > > --- a/drivers/virtio/virtio_vdpa.c
> > > +++ b/drivers/virtio/virtio_vdpa.c
> > > @@ -7,6 +7,7 @@
> > >   *
> > >   */
> > >  
> > > +#include "linux/virtio_config.h"
> > >  #include <linux/init.h>
> > >  #include <linux/module.h>
> > >  #include <linux/device.h>
> > > @@ -145,6 +146,7 @@ virtio_vdpa_setup_vq(struct virtio_device *vdev, unsigned int index,
> > >  	/* Assume split virtqueue, switch to packed if necessary */
> > >  	struct vdpa_vq_state state = {0};
> > >  	unsigned long flags;
> > > +	bool may_reduce_num = false;
> > >  	u32 align, num;
> > >  	int err;
> > >  
> > > @@ -169,10 +171,13 @@ virtio_vdpa_setup_vq(struct virtio_device *vdev, unsigned int index,
> > >  		goto error_new_virtqueue;
> > >  	}
> > >  
> > > +	if (ops->get_features(vdpa) & BIT_ULL(VIRTIO_F_VERSION_1))
> > > +		may_reduce_num = true;
> > > +
> > >  	/* Create the vring */
> > >  	align = ops->get_vq_align(vdpa);
> > >  	vq = vring_create_virtqueue(index, num, align, vdev,
> > > -				    true, true, ctx,
> > > +				    true, may_reduce_num, ctx,
> > >  				    virtio_vdpa_notify, callback, name);
> > >  	if (!vq) {
> > >  		err = -ENOMEM;
> > > diff --git a/include/uapi/linux/vdpa.h b/include/uapi/linux/vdpa.h
> > > index 66a41e4ec163..ce0b74276a5b 100644
> > > --- a/include/uapi/linux/vdpa.h
> > > +++ b/include/uapi/linux/vdpa.h
> > > @@ -32,6 +32,7 @@ enum vdpa_attr {
> > >  	VDPA_ATTR_DEV_VENDOR_ID,		/* u32 */
> > >  	VDPA_ATTR_DEV_MAX_VQS,			/* u32 */
> > >  	VDPA_ATTR_DEV_MAX_VQ_SIZE,		/* u16 */
> > > +	VDPA_ATTR_DEV_VERSION_1,		/* flag */
> > >  
> > >  	/* new attributes must be added above here */
> > >  	VDPA_ATTR_MAX,
> > > -- 
> > > 2.31.1