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

[Juergen Gross <jgross@xxxxxxxx>]

On 16.06.22 09:31, Oleksandr wrote:
> On 16.06.22 08:37, Juergen Gross wrote:
>
>
> Hello Juergen
>
> > 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.
> >
> > Fixes: fa1f57421e0b ("xen/virtio: Enable restricted memory access using Xen 
> > grant mappings")
> > Reported-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> > V2:
> > - remove command line parameter (Christoph Hellwig)
> > ---
> >   drivers/xen/Kconfig | 9 +++++++++
> >   include/xen/xen.h   | 2 +-
> >   2 files changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> > index bfd5f4f706bc..a65bd92121a5 100644
> > --- a/drivers/xen/Kconfig
> > +++ b/drivers/xen/Kconfig
> > @@ -355,4 +355,13 @@ config XEN_VIRTIO
> >         If in doubt, say n.
> > +config XEN_VIRTIO_FORCE_GRANT
> > +    bool "Require Xen virtio support to use grants"
> > +    depends on XEN_VIRTIO
> > +    help
> > +      Require virtio for Xen guests to use grant mappings.
> > +      This will avoid the need to give the backend the right to map all
> > +      of the guest memory. This will need support on the backend side
> > +      (e.g. qemu or kernel, depending on the virtio device types used).
> > +
> >   endmenu
> > diff --git a/include/xen/xen.h b/include/xen/xen.h
> > index 0780a81e140d..4d4188f20337 100644
> > --- a/include/xen/xen.h
> > +++ b/include/xen/xen.h
> > @@ -56,7 +56,7 @@ extern u64 xen_saved_max_mem_size;
> >   static inline void xen_set_restricted_virtio_memory_access(void)
> >   {
> > -    if (IS_ENABLED(CONFIG_XEN_VIRTIO) && xen_domain())
> > +    if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || xen_pv_domain())
> >           platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
>
> Looks like, the flag will be *always* set for paravirtualized guests even if 
> CONFIG_XEN_VIRTIO disabled.
> Maybe we should clarify the check?
>
>
> if (IS_ENABLED(CONFIG_XEN_VIRTIO_FORCE_GRANT) || IS_ENABLED(CONFIG_XEN_VIRTIO) 
> && xen_pv_domain())
>      platform_set(PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS);
Yes, we should. I had the function in grant-dma-ops.c in V1, and could drop the
CONFIG_XEN_VIRTIO dependency for that reason.

I'll wait for more comments before sending V3, though.


Juergen

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