Re: [PATCH] pc-bios/s390-ccw: Use memory barriers in virtio code

[Thomas Huth]

On 16/02/2021 12.47, Cornelia Huck wrote:
> On Tue, 16 Feb 2021 12:00:56 +0100
> Thomas Huth <thuth@redhat.com> wrote:
>
> > According to the virtio specification, a memory barrier should be
> > used before incrementing the idx field in the "available" ring.
> > So far, we did not do this in the s390-ccw bios yet, but recently
> > Peter Maydell saw problems with the s390-ccw bios when running
> > the qtests on an aarch64 host (the bios panic'ed with the message:
> > "SCSI cannot report LUNs: response VS RESP=09"), which could
> > maybe be related to the missing memory barriers. Thus let's add
> > those barriers now. Since we've only seen the problem on TCG so far,
> > a "bcr 14,0" should be sufficient here to trigger the tcg_gen_mb()
> > in the TCG translate code.
> >
> > (Note: The virtio spec also talks about using a memory barrier
> > *after* incrementing the idx field, but if I understood correctly
> > this is only required when using notification suppression - which
> > we don't use in the s390-ccw bios here)
> >
> > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > ---
> >   pc-bios/s390-ccw/virtio-net.c | 1 +
> >   pc-bios/s390-ccw/virtio.c     | 1 +
> >   pc-bios/s390-ccw/virtio.h     | 2 ++
> >   3 files changed, 4 insertions(+)
> >
> > diff --git a/pc-bios/s390-ccw/virtio-net.c b/pc-bios/s390-ccw/virtio-net.c
> > index 2fcb0a58c5..25598a7a97 100644
> > --- a/pc-bios/s390-ccw/virtio-net.c
> > +++ b/pc-bios/s390-ccw/virtio-net.c
> > @@ -127,6 +127,7 @@ int recv(int fd, void *buf, int maxlen, int flags)
> >   
> >       /* Mark buffer as available to the host again */
> >       rxvq->avail->ring[rxvq->avail->idx % rxvq->num] = id;
> > +    virtio_mb();
> >       rxvq->avail->idx = rxvq->avail->idx + 1;
> >       vring_notify(rxvq);
> >   
> > diff --git a/pc-bios/s390-ccw/virtio.c b/pc-bios/s390-ccw/virtio.c
> > index ab49840db8..fb9687f9b3 100644
> > --- a/pc-bios/s390-ccw/virtio.c
> > +++ b/pc-bios/s390-ccw/virtio.c
> > @@ -154,6 +154,7 @@ void vring_send_buf(VRing *vr, void *p, int len, int flags)
> >   
> >       /* Chains only have a single ID */
> >       if (!(flags & VRING_DESC_F_NEXT)) {
> > +        virtio_mb();
>
> I think you need to also need barriers for changes to the buffers, as
> the spec talks about "manipulating the descriptor table".

Which paragraph in the virtio spec are you refering to here? I can't find 
that part right now...

> >           vr->avail->idx++;
> >       }
> >   }
> > diff --git a/pc-bios/s390-ccw/virtio.h b/pc-bios/s390-ccw/virtio.h
> > index 19fceb6495..6ac65482a9 100644
> > --- a/pc-bios/s390-ccw/virtio.h
> > +++ b/pc-bios/s390-ccw/virtio.h
> > @@ -271,6 +271,8 @@ struct VirtioCmd {
> >   };
> >   typedef struct VirtioCmd VirtioCmd;
> >   
> > +#define virtio_mb()  asm volatile("bcr 14,0" : : : "memory")
>
> The bios is built for z900, so you probably need a bcr15 here?

I thought about that, too, but for TCG, it currently should not matter since 
both, 14 and 15, end up with the same code in op_bc() in 
target/s390x/translate.c. And on a real host, we've never seen this problem 
to occur, so it should not matter there, too. But if you prefer (e.g. in 
case somebody tweaks the TCG implementation one day), I can also switch to 
bcr15 instead.

 Thomas