On Thu, 25 Jun 2020 11:39:24 +0100 Daniel P. Berrangé <berra...@redhat.com> wrote:
> 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 <berra...@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. I did not find a call to virtio_pci_force_virtio_1(), so apparently not?
