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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jun 30, 2020 at 11:39:32AM +0200, Cornelia Huck wrote:
> On Tue, 30 Jun 2020 10:04:51 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
> 
> > On Mon, Jun 29, 2020 at 02:07:16PM +0100, Daniel P. Berrangé wrote:
> > > On Mon, Jun 29, 2020 at 01:53:05PM +0100, Stefan Hajnoczi 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)
> > > > > 
> > > > > A third option would be to refuse legacy altogether.  
> > > > 
> > > > I suggest the following:
> > > > 
> > > > 1. Combinations that worked with libvhost-user in the past must not break.
> > > > 
> > > > 2. New combinations should only support VIRTIO 1.0 and later.
> > > > 
> > > > This means continue to allow Legacy mode devices where they already run
> > > > today but don't add new code for the cases that didn't work.  
> > > 
> > > What I'm missing here is what PCI product ID was being used when the
> > > current impl is in legacy/transitional mode ?
> > > 
> > > Normally legacy and transitional mode devices need an explicit PCI ID
> > > reserved, where as modern-only devices have a PCI ID derived from their
> > > VirtIO ID + a fixed offset.
> > > 
> > > Was this mistakenly using a VirtIO ID + fixed offset for the legacy
> > > mode too ?  
> > 
> > vhost-user-fs-pci does not support Legacy or Transitional mode. See
> > hw/virtio/vhost-user-fs-pci.c:
> > 
> >   static const VirtioPCIDeviceTypeInfo vhost_user_fs_pci_info = {
> >       .base_name             = TYPE_VHOST_USER_FS_PCI,
> >       .non_transitional_name = "vhost-user-fs-pci",
> >       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >       .instance_size = sizeof(VHostUserFSPCI),
> >       .instance_init = vhost_user_fs_pci_instance_init,
> >       .class_init    = vhost_user_fs_pci_class_init,
> >   };
> 
> This makes it very unlikely that someone accidentally configures
> non-modern, but does not prevent it AFAICS. See
> <20200630113527.7b27f34f.cohuck@redhat.com>, which I just sent.
> 
> (I may be off, because that is all very confusing...)

Right. We'll block legacy for modern only devices going forward.
Going back to the patchset in question, virtio-fs is modern
only, legacy will not be supported.

-- 
MST