Re: [Xen-devel] [PATCH v17 00/23] x86/PMU: Xen PMU PV(H) support

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 01/20/2015 05:37 AM, Jan Beulich wrote:
> > > > On 05.01.15 at 22:43, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > Changes in v17:
> > * Disable VPMU when unknown CPU vendor is detected (patch #2)
> > * Remove unnecessary vendor tests in vendor-specific init routines (patch #14)
> > * Remember first CPU that starts mode change and use it to stop the cycle 
> > (patch #13)
> > * If vpmu ops is not present, return 0 as value for VPMU MSR read (as opposed to
> >    returning an error as was the case in previous patch.) (patch #18)
> Any particular reason for this?

PVH guests access MSR_IA32_MISC_ENABLE early during boot, before VPMU 
initialization code had a chance to run and set vpmu ops. Returning 1 
from vpmu_do_msr() would result in a fatal fault.
(This change would also be consistent with what core2_no_vpmu_ops does.)

I should have explained this in the cover letter.

-boris

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