Re: [Xen-devel] [PATCH 04/20] x86/shadow: Only apply shadow heuristics when in guest context

[Tim Deegan <tim@xxxxxxx>]

At 17:16 +0000 on 12 Feb (1423757763), Andrew Cooper wrote:
> It is incorrect to be applying these heuristics because of toolstack actions.

It might be incorrect if we did, but we don't. :P

The guess_wrmap() heuristics should only be called on v == current()
because they use the shadow linear map, but this patch actually
_relaxes_ that constraint so that they could run on other vcpus of the
same guest (harmlessly).

The second heuristic uses a stored smfn of a PTE, and uses the correct
callback for the shadow page's type, so is safe to call from any vcpu
so long as the domain's shadow lock is held.  The check you add here
is needed _only_ because you are changing from using 'v' to using
'curr' for the test.

The changes you've made in multi.c are again only needed because you
are s/v/curr/ in the same patch, and again if anything are relaxing
the checks.  Luckily they don't cover any mode-specific operations, so
AFAICT the code is still safe after all these changes, and at least no
worse than existing paths that use vcpu[0].

So this patch is safe, but it's not a bugfix, and I think the
changeset description needs to be rewritten.  What you're actually
doing is removing uses of the vcpu parameter in these functions,
replacing them with uses of current.  The changes to gating are a
side-effect.

With that change, Reviewed-by: Tim Deegan <tim@xxxxxxx>

Cheers,

Tim.

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