Re: [Xen-devel] [PATCH] RFC: Linux: disable APERF/MPERF feature in PV kernels

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.05.12 at 13:11, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 23/05/12 08:34, Jan Beulich wrote:
>> First of all I'm of the opinion that this indeed should not be
>> masked in the hypervisor - there's no reason to disallow the
>> guest to read these registers (but we should of course deny
>> writes as long as Xen is controlling P-states, which we do).
> 
> I am sorry but I am going to have to disagree with you on this point.
> 
> We should not be advertising this feature to any guest at all if we
> can't provide an implementation which works as native expects.  Else we
> are failing in our job of virtualisation.

That's perhaps a matter of the position you take - for HVM, I
would agree with yours, but there's many more aspects (not
the least related to accessing other MSRs) that we fail to
"properly" virtualize for PV guests - my position is that it is the
nature of PV that guest kernels have to be aware of being
virtualized (and hence stay away from doing certain things
unless [they think] they know what they're doing).

> There is 'dom0_vcpus_pin'[1] which identity pins dom0 vcpus, and
> prevents update of the affinity masks, and appears to conditionally
> allow access to certain MSRs.  I think it would be fine to expose this
> feature iff dom0s vcpus are pinned in this fashion.  That way, the
> measurement should succeed, even if dom0 only has read access to the MSRs.

Restricting it to this case would be too restrictive - it really
makes sense at any time where the vCPU's affinity has exactly
one bit set (or to be precise, the intersection of it and the set
of online pCPU-s).

Jan


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