Re: [PATCH v2] hw/s390x: Fix a possible crash with passed-through virtio devices

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Nov 18 2025, Halil Pasic <pasic@linux.ibm.com> wrote:

> Hm, the -EINVAL is put into GPR2 which is 'Host Cookie' according to the
> virtio specification:
> https://docs.oasis-open.org/virtio/virtio/v1.3/csd01/virtio-v1.3-csd01.html#x1-2260002
>
> Unfortunately, I did not find any words in the spec according to which
> GPR2 can be used to indicate errors. There does seem to be handling in
> the linux driver for that. It basically says negative is bad, but I can't
> see that in the spec. It just says "For each notification, the driver
> SHOULD use GPR4 to pass the host cookie received in GPR2 from the previous
> notification."
>
> Maybe we want to update the spec to reflect what is in the filed.

Saying that the driver SHOULD check GPR2 for negative error values is
probably fine, since it matches what is already out
there. (Unfortunately, we can't mandate it without either a new feature
bit or a new revision, and that would be overkill IMHO.)

For the device, we say "The device MAY return a 64-bit host cookie in
GPR2 to speed up the notification execution." -- the spec should
probably also say that the device MAY return a negative error code.