Date:   Wed, 10 Jun 2020 17:12:38 +0200
From:   Eugenio Perez Martin <eperezma@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm list <kvm@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH RFC v7 03/14] vhost: use batched get_vq_desc version

On Wed, Jun 10, 2020 at 5:08 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> 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 <mst@...hat.com>
> > > Signed-off-by: Eugenio Pérez <eperezma@...hat.com>
> > > Link: https://lore.kernel.org/r/20200401183118.8334-3-eperezma@redhat.com
> > > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > > ---
> > >  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/20200418102217.32327-1-eperezma@redhat.com/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?
>

Fields ndescs and first_descs are supposed to be updated outside, that
was the intention but maybe I forgot to delete it here, not sure.

Regarding avail_idx, the whole change has been tested with vhost_test
and for vhost needs to have a backend to modify it already so it seems
safe to me.

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