On Fri, 27 Jan 2017 13:24:13 +0100
Halil Pasic <pasic@xxxxxxxxxxxxxxxxxx> wrote:
> On 01/26/2017 08:20 PM, Michael S. Tsirkin wrote:
> > On Thu, Jan 26, 2017 at 06:39:14PM +0100, Halil Pasic wrote:
> >>
> >> Hi!
> >>
> >> Recently I have been investigating some strange migration problems on
> >> s390x.
> >>
> >> It turned out under certain circumstances vhost_net corrupts avail.idx by
> >> using wrong endianness.
>
> [..]
>
> >> -------------------------8<--------------
> >> >From b26e2bbdc03832a0204ee2b42967a1b49a277dc8 Mon Sep 17 00:00:00 2001
> >> From: Halil Pasic <pasic@xxxxxxxxxxxxxxxxxx>
> >> Date: Thu, 26 Jan 2017 00:06:15 +0100
> >> Subject: [PATCH] vhost: remove useless/dangerous reset of is_le
> >>
> >> The reset of is_le does no good, but it contributes its fair share to a
> >> bug in vhost_net, which occurs if we have some oldubufs when stopping and
> >> setting a fd = -1 as a backend. Instead of doing something convoluted in
> >> vhost_net, let's just get rid of the reset.
> >>
> >> Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxxxxxxx>
> >> Fixes: commit 2751c9882b94
> >> ---
> >> drivers/vhost/vhost.c | 4 +---
> >> 1 file changed, 1 insertion(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> >> index d643260..08072a2 100644
> >> --- a/drivers/vhost/vhost.c
> >> +++ b/drivers/vhost/vhost.c
> >> @@ -1714,10 +1714,8 @@ int vhost_vq_init_access(struct vhost_virtqueue *vq)
> >> int r;
> >> bool is_le = vq->is_le;
> >>
> >> - if (!vq->private_data) {
> >> - vhost_reset_is_le(vq);
> >> + if (!vq->private_data)
> >> return 0;
> >> - }
> >>
> >> vhost_init_is_le(vq);
> >
> >
> > I think you do need to reset it, just maybe within vhost_init_is_le.
> >
> > if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> > vq->is_le = true;
> > else
> > vhost_reset_is_le(vq);
> >
> >
>
> That is a very good point! I have overlooked that while the
> CONFIG_VHOST_CROSS_ENDIAN_LEGACY variant
>
> static void vhost_init_is_le(struct vhost_virtqueue *vq)
> {
> /* Note for legacy virtio: user_be is initialized at reset time
> * according to the host endianness. If userspace does not set an
> * explicit endianness, the default behavior is native endian, as
> * expected by legacy virtio.
> */
> vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) || !vq->user_be;
> }
>
> is fine the other variant
>
> static void vhost_init_is_le(struct vhost_virtqueue *vq)
> {
> if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> vq->is_le = true;
> }
> is a very strange initializer (makes assumptions about the state
> to be initialized).
>
> I agree, setting native endianness there sounds very reasonable.
>
> I have a question regarding readability. IMHO the relationship
> of reset_is_le and int_is_le is a bit confusing, and I'm afraid
> it could become even more confusing with using reset in one of
> the init_is_le's.
>
> How about we do the following?
>
> static void vhost_init_is_le(struct vhost_virtqueue *vq)
> {
> if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
> vq->is_le = true;
> + else
> + vq->is_le = virtio_legacy_is_little_endian();
>
> }
>
> static void vhost_reset_is_le(struct vhost_virtqueue *vq)
> {
> - vq->is_le = virtio_legacy_is_little_endian();
> + vhost_init_is_le(vq);
> }
>
> That way we would have correct endianness both after reset
> and after init, I think :).
>
Yes, I think this is what we need.
Cheers.
--
Greg
> Thank you very much!
>
> Halil
>
>
