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

[Peter Maydell]

On Tue, 16 Feb 2021 at 14:35, Thomas Huth <thuth@redhat.com> wrote:
>
> On 16/02/2021 15.32, Peter Maydell wrote:
> > On Tue, 16 Feb 2021 at 14:30, Cornelia Huck <cohuck@redhat.com> wrote:
> >> Step 4 in "2.7.13 Supplying Buffers to The Device":
> >>
> >> "The driver performs a suitable memory barrier to ensure the device
> >> sees the updated descriptor table and available ring before the next
> >> step."
> >
> > I thought that my first time through the spec as well, but
> > I think the whole of section 2.7 is dealing with "packed virtqueues",
> > which have to be explicitly negotiated and which I don't think
> > the s390-ccw code is doing.
>
> Right. I think the s390-ccw code is still based on virtio v1.0, that's why I
> also only looked at that version of the spec.

I think the ideal would be to find somebody who's really well
acquainted with the virtio spec (MST?) and ask them to have
a quick look-over the s390-ccw code to say where it needs
changes... The fact that this patch doesn't completely fix
the bug leaves me suspicious that we're missing something in
our readings of the spec.

-- PMM