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

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 28 Jul 2020 12:48:50 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> On Tue, 21 Jul 2020 18:44:56 +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.
> 
> Can someone please clarify what libvhost-user actually supports?
> 
> virtio-fs is clearly non-legacy only, but in the changelog I also see a
> mention of scsi, and that one probably does support legacy?
> 
> Can we make a distinction along the lines of:
> - if we are non-legacy, we support any endianness;
> - if we are legacy, we support little endian host/guest only?
> 

I don't like the idea of this conditional support of legacy (with a
condition being a little endian host/guest). The library is supposed
to be portable in my opinion.

Regards,
Halil