Re: [PATCH RFC] virtio-fs: force virtio 1.x usage

[Michael S. Tsirkin]

On Wed, Jul 01, 2020 at 03:58:19PM +0200, Cornelia Huck wrote:
> On Tue, 30 Jun 2020 09:02:31 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Tue, Jun 30, 2020 at 01:30:43PM +0200, Cornelia Huck wrote:
> > > On Tue, 30 Jun 2020 06:45:42 -0400
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > >   
> > > > On Tue, Jun 30, 2020 at 11:35:27AM +0200, Cornelia Huck wrote:  
> 
> > > > > First, I noticed that virtio-iommu does not force virtio-1, either; I
> > > > > think it should? Eric?
> > > > > 
> > > > > Then, there's the mechanism using different names for transitional and
> > > > > non-transitional devices. Devices that support both usually define 
> > > > > both
> > > > > names (with disable_legacy and disable_modern set appropriately) and a
> > > > > base name (where the properties can be set manually for the desired
> > > > > effect). Most virtio-1 only devices set neither the non-transitional
> > > > > nor the transitional name and rely on virtio_pci_force_virtio_1() to
> > > > > disable legacy support. But there are outliers:
> > > > > 
> > > > > * this device: it has only a non-transitional name
> > > > >   ("vhost-user-fs-pci"), which means we automatically get the correct
> > > > >   configuration; in order to define a transitional/legacy device, you
> > > > >   would need to use the base name "vhost-user-fs-pci-base" explicitly,
> > > > >   and it's unlikely that someone has been doing that.
> > > > > * virtio-iommu (which I *think* is a virtio-1 only device): it defines
> > > > >   the full set of transitional, non-transitional, and base names.
> > > > > 
> > > > > How should we proceed?
> > > > > * With this patch here, we can fence off the very unlikely possibility
> > > > >   of somebody configuring a non-modern virtio-fs device for pci. We
> > > > >   probably should do it, but I don't think we need compat handling.
> > > > > * For virtio-iommu, we should get an agreement what the desired state
> > > > >   is. If it really should be modern only, we need compat handling, as
> > > > >   the device had been added in 5.0. (And we need to figure out how to
> > > > >   apply that compat handling.)    
> > > > 
> > > > 
> > > > Well I know it's not really used on x86 yet, so no problem there.
> > > > 
> > > > Which machines are actually affected?  
> > > 
> > > I'd suspect ARM, but breaking even a subset is not nice.  
> > 
> > OK so MMIO does not have transitional at all right?
> 
> IIRC, yes.
> 
> But I think there are ARM machines that use virtio-pci as well, right?


Right :(

I guess we do need a compat property for that.


-- 
MST