Re: [PATCH 2/2] pc-bios/s390-ccw/virtio: Read device config after feature negotiation

[Cornelia Huck <cohuck@redhat.com>]

On Thu, Jun 23 2022, Thomas Huth <thuth@redhat.com> wrote:

> On 23/06/2022 10.44, Cornelia Huck wrote:
>> On Thu, Jun 23 2022, Thomas Huth <thuth@redhat.com> wrote:
>> 
>>> Feature negotiation should be done first, since some fields in the
>>> config area can depend on the negotiated features and thus should
>>> rather be read afterwards.
>> 
>> I suppose we don't negotiate any features that might affect the size of
>> the config space? Anyway, restricting ourselves to the minimum length
>> should be fine.
>
> Actually, even the virtio spec 0.9.5 already talks about 
> VIRTIO_BLK_F_BLK_SIZE and VIRTIO_BLK_F_GEOMETRY being necessary to get the 
> corresponding values in the config space ... so we're currently depending on 
> the good will of QEMU to also provide the values without these feature bits. 

Eww. The bad thing about QEMU being so relaxed is that you don't catch
issues like that... but if it works for now, there's at least no
pressure.

> I'm already thinking about providing a patch to properly request these 
> feature bits in the s390-ccw bios ... but the whole code there is so ugly 
> that I need some time to think about the right steps to clean it up first.

It's not ugly, it has grown organically :) (just like that overgrown
spot in my garden ;)