Subject: Re: [PATCH RFC v7 03/14] vhost: use batched get_vq_desc version

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 10 Jun 2020 11:08:02 -0400

On Wed, Jun 10, 2020 at 04:29:29PM +0200, Eugenio Pérez wrote:
> On Wed, 2020-06-10 at 07:36 -0400, Michael S. Tsirkin wrote:
> > As testing shows no performance change, switch to that now.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > Signed-off-by: Eugenio Pérez <[email-masked]>
> > Link: https://lore.kernel.org/r/[email-masked]
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >  drivers/vhost/test.c  |   2 +-
> >  drivers/vhost/vhost.c | 318 ++++++++----------------------------------
> >  drivers/vhost/vhost.h |   7 +-
> >  3 files changed, 65 insertions(+), 262 deletions(-)
> > 
> > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > index 0466921f4772..7d69778aaa26 100644
> > --- a/drivers/vhost/test.c
> > +++ b/drivers/vhost/test.c
> > @@ -119,7 +119,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
> >  	dev = &n->dev;
> >  	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
> >  	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
> > -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
> > +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
> >  		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT, true, NULL);
> >  
> >  	f->private_data = n;
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 11433d709651..28f324fd77df 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -304,6 +304,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> >  {
> >  	vq->num = 1;
> >  	vq->ndescs = 0;
> > +	vq->first_desc = 0;
> >  	vq->desc = NULL;
> >  	vq->avail = NULL;
> >  	vq->used = NULL;
> > @@ -372,6 +373,11 @@ static int vhost_worker(void *data)
> >  	return 0;
> >  }
> >  
> > +static int vhost_vq_num_batch_descs(struct vhost_virtqueue *vq)
> > +{
> > +	return vq->max_descs - UIO_MAXIOV;
> > +}
> > +
> >  static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
> >  {
> >  	kfree(vq->descs);
> > @@ -394,6 +400,9 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
> >  	for (i = 0; i < dev->nvqs; ++i) {
> >  		vq = dev->vqs[i];
> >  		vq->max_descs = dev->iov_limit;
> > +		if (vhost_vq_num_batch_descs(vq) < 0) {
> > +			return -EINVAL;
> > +		}
> >  		vq->descs = kmalloc_array(vq->max_descs,
> >  					  sizeof(*vq->descs),
> >  					  GFP_KERNEL);
> > @@ -1610,6 +1619,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> >  		vq->last_avail_idx = s.num;
> >  		/* Forget the cached index value. */
> >  		vq->avail_idx = vq->last_avail_idx;
> > +		vq->ndescs = vq->first_desc = 0;
> 
> This is not needed if it is done in vhost_vq_set_backend, as far as I can tell.
> 
> Actually, maybe it is even better to move `vq->avail_idx = vq->last_avail_idx;` line to vhost_vq_set_backend, it is part
> of the backend "set up" procedure, isn't it?
> 
> I tested with virtio_test + batch tests sent in 
> https://lkml.kernel.org/lkml/[email-masked]/T/.

Ow did I forget to merge them for rc1?  Should I have? Maybe Linus won't
yell to hard at me if I merge them after rc1.

> I append here what I'm proposing in case it is clearer this way.
> 
> Thanks!
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 4d198994e7be..809ad2cd2879 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1617,9 +1617,6 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
>  			break;
>  		}
>  		vq->last_avail_idx = s.num;
> -		/* Forget the cached index value. */
> -		vq->avail_idx = vq->last_avail_idx;
> -		vq->ndescs = vq->first_desc = 0;
>  		break;
>  	case VHOST_GET_VRING_BASE:
>  		s.index = idx;
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index fed36af5c444..f4902dc808e4 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -258,6 +258,7 @@ static inline void vhost_vq_set_backend(struct vhost_virtqueue *vq,
>  					void *private_data)
>  {
>  	vq->private_data = private_data;
> +	vq->avail_idx = vq->last_avail_idx;
>  	vq->ndescs = 0;
>  	vq->first_desc = 0;
>  }
> 

Seems like a nice cleanup, though it's harmless right?

-- 
MST