Re: [PATCH 0/6] vhost: Reset batched descriptors on SET_VRING_BASE call

[Eugenio Perez Martin <eperezma@redhat.com>]

On Sun, Mar 29, 2020 at 1:49 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Sun, Mar 29, 2020 at 01:33:53PM +0200, Eugenio Pérez wrote:
> > Vhost did not reset properly the batched descriptors on SET_VRING_BASE event. Because of that, is possible to return an invalid descriptor to the guest.
> > This series ammend this, and creates a test to assert correct behavior. To do that, they need to expose a new function in virtio_ring, virtqueue_reset_free_head. Not sure if this can be avoided.
>
> Question: why not reset the batch when private_data changes?
> At the moment both net and scsi poke at private data directly,
> if they do this through a wrapper we can use that to
> 1. check that vq mutex is taken properly
> 2. reset batching
>
> This seems like a slightly better API
>

I didn't do that way because qemu could just SET_BACKEND to -1 and
SET_BACKEND to the same one, with no call to SET_VRING. In this case,
I think that qemu should not change the descriptors already pushed. I
do agree with the interface to modify private_data properly (regarding
the mutex).

However, I can see how your proposal is safer, so we don't even need
to check if private_data is != NULL when we have descriptors in the
batch_descs array. Also, this ioctls should not be in the hot path, so
we can change to that mode anyway.

> >
> > Also, change from https://lkml.org/lkml/2020/3/27/108 is not included, that avoids to update a variable in a loop where it can be updated once.
> >
> > This is meant to be applied on top of eccb852f1fe6bede630e2e4f1a121a81e34354ab in git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git, and some commits should be squashed with that series.
>
> Thanks a lot! I'll apply this for now so Christian can start testing,
> but I'd like the comment above addressed before I push this to Linus.
>
> > Eugenio Pérez (6):
> >   tools/virtio: Add --batch option
> >   tools/virtio: Add --batch=random option
> >   tools/virtio: Add --reset=random
> >   tools/virtio: Make --reset reset ring idx
> >   vhost: Delete virtqueue batch_descs member
> >   fixup! vhost: batching fetches
> >
> >  drivers/vhost/test.c         |  57 ++++++++++++++++
> >  drivers/vhost/test.h         |   1 +
> >  drivers/vhost/vhost.c        |  12 +++-
> >  drivers/vhost/vhost.h        |   1 -
> >  drivers/virtio/virtio_ring.c |  18 +++++
> >  include/linux/virtio.h       |   2 +
> >  tools/virtio/linux/virtio.h  |   2 +
> >  tools/virtio/virtio_test.c   | 123 +++++++++++++++++++++++++++++++----
> >  8 files changed, 201 insertions(+), 15 deletions(-)
> >
> > --
> > 2.18.1
>