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



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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