Re: [PATCH v2] xen: don't require virtio with grants for non-PV guests

[Juergen Gross]

On 16.06.22 08:03, Christoph Hellwig wrote:
> On Thu, Jun 16, 2022 at 07:37:15AM +0200, Juergen Gross wrote:
> > Commit fa1f57421e0b ("xen/virtio: Enable restricted memory access using
> > Xen grant mappings") introduced a new requirement for using virtio
> > devices: the backend now needs to support the VIRTIO_F_ACCESS_PLATFORM
> > feature.
> >
> > This is an undue requirement for non-PV guests, as those can be operated
> > with existing backends without any problem, as long as those backends
> > are running in dom0.
> >
> > Per default allow virtio devices without grant support for non-PV
> > guests.
> >
> > Add a new config item to always force use of grants for virtio.
>
> What Í'd really expect here is to only set the limitations for the
> actual grant-based devic.  Unfortunately
> PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS is global instead of per-device,

I think the global setting is fine, as it serves a specific purpose:
don't allow ANY virtio devices without the special handling (like the
/390 PV case, SEV, TDX, or Xen PV-guests). Those cases can't sensibly
work without the special DMA ops.

In case the special DMA ops are just a "nice to have" like for Xen HVM
guests, PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS won't be set.

And if someone wants a guest only to use grant based virtio devices,
the guest kernel can be built with CONFIG_XEN_VIRTIO_FORCE_GRANT (e.g.
in case the backends are running in some less privileged environment
and thus can't map arbitrary guest memory pages).


Juergen

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