Re: [RFC 0/4] Enable virtio-fs on s390x

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Thu, Jun 25, 2020 at 12:04:26PM +0200, Marc Hartmayer wrote:
> > This RFC is about enabling virtio-fs on s390x. For that we need
> >  + some shim code (first patch), and we need
> >  + libvhost-user to deal with virtio endiannes as mandated by the spec.
> >  
> > The second part is trickier, because unlike QEMU we are not certain
> > about the guest's native endianness, which is needed to handle the
> > legacy-interface appropriately. In fact, this is the reason why just
> > RFC.
> > 
> > One of the open questions is whether to build separate versions, one
> > for guest little endian and one for guest big endian, or do we want
> > something like a command line option? (Digression on the libvirt
> > modeling)
> 
> When you talk about  big vs little endian, are you referring to TCG
> scenarios with mixed host/guest arch, or arches which can support
> either endianess, or both ? i guess it doesn't matter actually, as
> I think the latter forces a specific answer.
> 
> Considering that some architectures allow the guest OS to flip between
> big & little endian as they boot, libvirt cannot know what endianess
> the guest is using when it launches virtiofsd. It thus cannot pick
> between two different endianness builds of virtiofsd automatically.
> This would force the user to tell libvirt what arch the guest is using
> at the time they define the guest. This is an undesirable restriction
> for use cases where the admin of the guest OS has no direct control
> over the host config.
> 
> IOW, I think the only practical answer is to have a single binary that
> automagically does the right thing at runtime according to guest
> endianess that currently is in use.

Modifying the fuse code in virtiofsd to handle cross-endian would be a
big job;  IMHO unless you really have a cross-endian need I'd just make
sure it deals ok with big<->big and little<->little.

Dave

> Regards,
> Daniel
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK