Re: [Xen-devel] [PATCH RFC v2 1/3] xen/pvh: enable mmu_update hypercall

[Jürgen Groß <jgross@xxxxxxxx>]

On 04/02/2015 01:50 PM, Andrew Cooper wrote:
> On 02/04/15 11:42, Ian Campbell wrote:
> > On Thu, 2015-04-02 at 12:26 +0200, Roger Pau Monne wrote:
> > > This is needed for performing save/restore of PV guests.
> > It's quite a big interface though, isn't it?
> >
> > Could we restrict it to a subset of the operations perhaps? Or at least
> > justify here how it has been audited and found to be safe to allow an
> > HVM guest this access.
> It isn't actually very big, but does have quite a lot of PV knowledge
> built in.
>
> I would be happer with the safety of this patch if
> v->arch.old_guest_table got moved into the pv union, to make the code
> much clearer that it is specifically for PV guests.
>
> If I recall, this change only needed for MMU_MACHPHYS_UPDATE against a
> foreign domain.  Each of the 3 subops does check for
> paging_mode_translate/refcounts() of the target, which does prevent the
> hypercall being made against a non-PV domains.  From that point of view,
> it should be safe for HVM guests to use, as it is the target domain,
> rather than the source domain, which is important.
>
>
> However, with migration v2 dropping support for 2-level PV guests (which
> died with the 32bit hypervisor build), I believe I have removed all need
> for MMU_MACHPHYS_UPDATE hypercalls entirely (unless there are some done
> behind the scenes from the kernel).
There is one usage in the kernel in file arch/x86/xen/setup.c targeting
DOMID_SELF.


Juergen

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