[PATCH] KVM: arm64: Ignore PMCNTENSET_EL0 while checking for overflow status

[PATCH] KVM: arm64: Ignore PMCNTENSET_EL0 while checking for overflow status

[Raghavendra Rao Ananta]

kvm_pmu_overflow_status() currently checks if the PMCs are enabled for
evaluating the PMU overflow condition. However, ARM ARM D13.1.1 states
that a global enable control (PMCR.E), PMOVSSET<n>, and PMINTENSET<n>
are sufficent to consider that the overflow condition is met. Hence,
ignore the check for PMCNTENSET<n>.

The bug was discovered while running the SBSA PMU test, which only sets
PMCR.E, PMOVSSET<0>, PMINTENSET<0>, and expects an overflow interrupt.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
---
 arch/arm64/kvm/pmu-emul.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index ac36c438b8c1..3940fe893783 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -342,7 +342,6 @@ static u64 kvm_pmu_overflow_status(struct kvm_vcpu *vcpu)
 
 	if ((kvm_vcpu_read_pmcr(vcpu) & ARMV8_PMU_PMCR_E)) {
 		reg = __vcpu_sys_reg(vcpu, PMOVSSET_EL0);
-		reg &= __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
 		reg &= __vcpu_sys_reg(vcpu, PMINTENSET_EL1);
 	}
 

base-commit: adc218676eef25575469234709c2d87185ca223a
-- 
2.47.0.338.g60cca15819-goog

Re: [PATCH] KVM: arm64: Ignore PMCNTENSET_EL0 while checking for overflow status

[Oliver Upton]

Hi Raghu,

Thanks for finding this!

On Tue, Nov 19, 2024 at 08:58:41PM +0000, Raghavendra Rao Ananta wrote:
> kvm_pmu_overflow_status() currently checks if the PMCs are enabled for
> evaluating the PMU overflow condition. However, ARM ARM D13.1.1 states
> that a global enable control (PMCR.E), PMOVSSET<n>, and PMINTENSET<n>
> are sufficent to consider that the overflow condition is met. Hence,
> ignore the check for PMCNTENSET<n>.

It's more than sufficient, evaluating E, PMOVSSET<n>, and PMINTENSET<n>
is the *only* correct implementation of the architecture.

Also, ARM ARM section numbering is subject to change between revisions,
so it's always best to use a fully-qualified citation, like 'DDI0487K
D13.1.1'.

So I may rewrite this as:

  DDI0487K D13.1.1 describes the PMU overflow condition, which evaluates
  to true if any counter's global enable (PMCR_EL0.E), overflow flag
  (PMOVSSET_EL0[n]), and interrupt enable (PMINTENSET_EL1[n]) are all 1.
  Of note, this does not require a counter to be enabled (i.e.
  PMCNTENSET_EL0[n] = 1) to generate an overflow.

  Align kvm_pmu_overflow_status() with the reality of the architecture
  and stop using PMCNTENSET_EL0 as part of the overflow condition.

We've got yet another bug lurking here as of 6.13, since the hypervisor
range of counters isn't observing the correct global enable
(MDCR_EL2.HPME).

Let me fiddle with this and send out a combined set of fixes.

> The bug was discovered while running the SBSA PMU test, which only sets
> PMCR.E, PMOVSSET<0>, PMINTENSET<0>, and expects an overflow interrupt.
> 

We should be sending this to stable too.

Cc: stable@vger.kernel.org
Fixes: 76d883c4e640 ("arm64: KVM: Add access handler for PMOVSSET and PMOVSCLR register")

-- 
Thanks,
Oliver