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

[Juergen Gross <jgross@xxxxxxxx>]

On 16.06.22 22:33, Oleksandr wrote:
> On 16.06.22 11:56, Juergen Gross wrote:
>
> Hello Juergen, all
>
>
> > 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.
> ok
>
>
>
> Please note, I am happy with current patch and it works in my Arm64 based 
> environment.
> Just one moment to consider.
>
>
> As it was already mentioned earlier in current thread the 
> PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS (former 
> arch_has_restricted_virtio_memory_access()) is not per device but about the 
> whole guest. Being set it makes VIRTIO_F_ACCESS_PLATFORM and VIRTIO_F_VERSION_1 
> features mandatory for *all* virtio devices in the guest.
> The question is “Do we want/need to lift this restriction for some devices 
> (which backends are trusted so can access all guest memory) at the same time”? 
No, if you need some virtio devices to not use grants, then don't set
PLATFORM_VIRTIO_RESTRICTED_MEM_ACCESS.

Please see my answer to Stefano's alternative solution for my idea how to
resolve this via a per-device setting.


Juergen

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