Re: [Xen-devel] patch for restricted vPMU modes

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/23/2015 05:01 PM, Brendan Gregg wrote:
> On Mon, Nov 23, 2015 at 6:35 AM, Boris Ostrovsky 
> <boris.ostrovsky@xxxxxxxxxx <mailto:boris.ostrovsky@xxxxxxxxxx>> wrote:
>
>     On 11/20/2015 07:21 PM, Brendan Gregg wrote:
>
>     These new features would need a corresponding change in Linux for
>     PV guests (or for dom0 to change feature set globally). But before
>     that do_xenpmu_op()'s XENPMU_feature_set clause will have to be
>     updated to deal with new modes.
>
>
> Ok, thanks. For now this is HVM, and I can EINVAL in do_xenpmu_op() 
> for the new modes, like with BTS. (Do you know if the later Linux 
> changes would be more than a feature version of 
> pmu_mode_store/pmu_mode_show?)

You are right --- Linux doesn't need anything, it just passes the 
features directly to the hypervisor. pmu_mode_store/show convert strings 
to numbers but we don't use strings for features.
So the only thing that's needed is to allow hypervisor set/clear those 
two bits in vpmu_features in do_xenpmu_op().

>     Also, if either of those two flags is set we probably want to
>     block MSR_IA32_DS_AREA and MSR_IA32_PEBS_ENABLE accesses as well.
>
>
> Ok, yes, good idea.

But: DS area is needed by BTS so if that feature is set we want to allow 
guests to write it (and as for PEBS (which also uses DS area) --- that's 
really a nop so whether or not you block it won't make any difference. 
But we should probably still do it).
I don't know whether anyone else uses debug store. The way SDM's section 
18.11.4 describes it there doesn't seem to be any other users.
-boris



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