Re: [PATCH] KVM: x86/pmu: Fix type length error when reading pmu->fixed_ctr_ctrl

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 01, 2024, Mingwei Zhang wrote:
> On Thu, Feb 01, 2024, Sean Christopherson wrote:
> > On Wed, Jan 31, 2024, Mingwei Zhang wrote:
> > > > The PMC is still active while the VM side handle_pmi_common() is not going to handle it?
> > > 
> > > hmm, so the new value is '0', but the old value is non-zero, KVM is
> > > supposed to zero out (stop) the fix counter), but it skips it. This
> > > leads to the counter continuously increasing until it overflows, but
> > > guest PMU thought it had disabled it. That's why you got this warning?
> > 
> > No, that can't happen, and KVM would have a massive bug if that were the case.
> > The truncation can _only_ cause bits to disappear, it can't magically make bits
> > appear, i.e. the _only_ way this can cause a problem is for KVM to incorrectly
> > think a PMC is being disabled.
> 
> The reason why the bug does not happen is because there is global
> control. So disabling a counter will be effectively done in the global
> disable part, ie., when guest PMU writes to MSR 0x38f.


> > fixed PMC is disabled. KVM will pause the counter in reprogram_counter(), and
> > then leave the perf event paused counter as pmc_event_is_allowed() will return
> > %false due to the PMC being locally disabled.
> >
> > But in this case, _if_ the counter is actually enabled, KVM will simply reprogram
> > the PMC.  Reprogramming is unnecessary and wasteful, but it's not broken.
> 
> no, if the counter is actually enabled, but then it is assigned to
> old_fixed_ctr_ctrl, the value is truncated. When control goes to the
> check at the time of disabling the counter, KVM thinks it is disabled,
> since the value is already truncated to 0. So KVM will skip by saying
> "oh, the counter is already disabled, why reprogram? No need!".

Ooh, I had them backwards.  KVM can miss 1=>0, but not 0=>1.  I'll apply this
for 6.8; does this changelog work for you?

  Use a u64 instead of a u8 when taking a snapshot of pmu->fixed_ctr_ctrl
  when reprogramming fixed counters, as truncating the value results in KVM
  thinking all fixed counters, except counter 0, are already disabled.  As
  a result, if the guest disables a fixed counter, KVM will get a false
  negative and fail to reprogram/disable emulation of the counter, which can
  leads to spurious PMIs in the guest.