Re: [RFC v2 2/3] libvhost-user: handle endianness as mandated by the spec

[Michael S. Tsirkin]

On Tue, Jul 28, 2020 at 12:52:11PM +0200, Marc Hartmayer wrote:
> On Tue, Jul 21, 2020 at 06:44 PM +0200, Halil Pasic <pasic@linux.ibm.com> 
> wrote:
> > On Tue, 21 Jul 2020 09:40:10 -0400
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >
> >> On Fri, Jul 17, 2020 at 11:29:28AM +0200, Marc Hartmayer wrote:
> >> > Since virtio existed even before it got standardized, the virtio
> >> > standard defines the following types of virtio devices:
> >> > 
> >> >  + legacy device (pre-virtio 1.0)
> >> >  + non-legacy or VIRTIO 1.0 device
> >> >  + transitional device (which can act both as legacy and non-legacy)
> >> > 
> >> > Virtio 1.0 defines the fields of the virtqueues as little endian,
> >> > while legacy uses guest's native endian [1]. Currently libvhost-user
> >> > does not handle virtio endianness at all, i.e. it works only if the
> >> > native endianness matches with whatever is actually needed. That means
> >> > things break spectacularly on big-endian targets. Let us handle virtio
> >> > endianness for non-legacy as required by the virtio specification
> >> > [1]. We will fence non-legacy virtio devices with the upcoming patch.
> >> > 
> >> > [1] 
> >> > https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-210003
> >> > 
> >> > Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
> >> > 
> >> > ---
> >> > Note: As we don't support legacy virtio devices  
> >> 
> >> Who's "we" in this sentence? vhost user supports legacy generally ...
> >
> > In that sentence "we" is the library "libvhost-user". I would like
> > to avoid s390x being an oddball regarding this. Marc's previous
> > version made an attempt at correct endianness handling for legacy
> > and non-legacy. That spawned a discussion on how we don't want
> > legacy devices in this context. This series makes what seemed to be the
> > consensus reached in that discussion explicit: namely that libvhost-user
> > does not support legacy-virtio.
> 
> Hi Michael,
> 
> Polite ping - what’s your opinion? Thanks in advance.
> 
> […snip]

It's a reasonable limitation, but I also don't see anything
that verifies that device is modern only.
E.g. fail setting features if VIRTIO_F_VERSION_1 is not there?


> -- 
> Kind regards / Beste Grüße
>    Marc Hartmayer
> 
> IBM Deutschland Research & Development GmbH
> Vorsitzender des Aufsichtsrats: Gregor Pillen 
> Geschäftsführung: Dirk Wittkopp
> Sitz der Gesellschaft: Böblingen
> Registergericht: Amtsgericht Stuttgart, HRB 243294