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

[Daniel P . Berrangé]

On Thu, Jun 25, 2020 at 12:31:36PM +0200, Cornelia Huck wrote:
> 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?

I'd hope virtio-fs is already forced to modern only, as there's no legacy
PCI ID assigned to it in the spec.

Regards,
Daniel
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