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

[Cornelia Huck]

On Thu, 25 Jun 2020 11:19:35 +0100
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.

Right, but that is in practice only a problem for legacy devices, isn't
it? The standard says that non-legacy devices use little-endian
everywhere; it's the legacy 'device endian' that is causing us
headaches.

Which leads to the question: Do we really need to support legacy
virtio-fs devices, or can we just force virtio-1, as many (most?) newer
virtio devices do?

> 
> 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.
> 
> Regards,
> Daniel