Re: [PATCH v6 04/16] KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE_EMON bit when vPMU is enabled

[Sean Christopherson]

On Mon, May 17, 2021, Sean Christopherson wrote:
> On Thu, May 13, 2021, Xu, Like wrote:
> > On 2021/5/12 23:18, Sean Christopherson wrote:
> > > On Wed, May 12, 2021, Xu, Like wrote:
> > > > Hi Venkatesh Srinivas,
> > > > 
> > > > On 2021/5/12 9:58, Venkatesh Srinivas wrote:
> > > > > On 5/10/21, Like Xu <like.xu@xxxxxxxxxxxxxxx> wrote:
> > > > > > On Intel platforms, the software can use the IA32_MISC_ENABLE[7] bit to
> > > > > > detect whether the processor supports performance monitoring facility.
> > > > > > 
> > > > > > It depends on the PMU is enabled for the guest, and a software write
> > > > > > operation to this available bit will be ignored.
> > > > > Is the behavior that writes to IA32_MISC_ENABLE[7] are ignored (rather than #GP)
> > > > > documented someplace?
> > > > The bit[7] behavior of the real hardware on the native host is quite
> > > > suspicious.
> > > Ugh.  Can you file an SDM bug to get the wording and accessibility updated?  The
> > > current phrasing is a mess:
> > > 
> > >    Performance Monitoring Available (R)
> > >    1 = Performance monitoring enabled.
> > >    0 = Performance monitoring disabled.
> > > 
> > > The (R) is ambiguous because most other entries that are read-only use (RO), and
> > > the "enabled vs. disabled" implies the bit is writable and really does control
> > > the PMU.  But on my Haswell system, it's read-only.
> > 
> > On your Haswell system, does it cause #GP or just silent if you change this
> > bit ?
> 
> Attempting to clear the bit generates a #GP.

*sigh*

Venkatesh and I are exhausting our brown paper bag supply.

Attempting to clear bit 7 is ignored on both Haswell and Goldmont.  This _no_ #GP,
the toggle is simply ignored.  I forgot to specify hex format (multiple times),
and Venkatesh accessed the wrong MSR (0x10a instead of 0x1a0).

So your proposal to ignore the toggle in KVM is the way to go, but please
document in the changelog that that behavior matches bare metal.

It would be nice to get the SDM cleaned up to use "supported/unsupported", and to
pick one of (R), (RO), and (R/O) for all MSRs entries for consistency, but that
may be a pipe dream.

Sorry for the run-around :-/