[PATCH v2] arm64: KVM: pmu: Reset PMSELR_EL0.SEL to a sane value before entering the guest

The ARMv8 architecture allows the cycle counter to be configured
by setting PMSELR_EL0.SEL==0x1f and then accessing PMXEVTYPER_EL0,
hence accessing PMCCFILTR_EL0. But it disallows the use of
PMSELR_EL0.SEL==0x1f to access the cycle counter itself through
PMXEVCNTR_EL0.

Linux itself doesn't violate this rule, but we may end up with
PMSELR_EL0.SEL being set to 0x1f when we enter a guest. If that
guest accesses PMXEVCNTR_EL0, the access may UNDEF at EL1,
despite the guest not having done anything wrong.

In order to avoid this unfortunate course of events (haha!), let's
sanitize PMSELR_EL0 on guest entry. This ensures that the guest
won't explode unexpectedly.

Signed-off-by: Marc Zyngier <redacted>
---
This is another approach to fix this issue, this time nuking PMSELR_EL0
on guest entry instead of relying on perf not to clobber the register.

Tested on v4.9-rc8 with a Rev A3 X-Gene.

 arch/arm64/kvm/hyp/switch.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 83037cd..3b7cfbd 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c

@@ -85,7 +85,13 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
 	write_sysreg(val, hcr_el2);
 	/* Trap on AArch32 cp15 c15 accesses (EL1 or EL0) */
 	write_sysreg(1 << 15, hstr_el2);
-	/* Make sure we trap PMU access from EL0 to EL2 */
+	/*
+	 * Make sure we trap PMU access from EL0 to EL2. Also sanitize
+	 * PMSELR_EL0 to make sure it never contains the cycle
+	 * counter, which could make a PMXEVCNTR_EL0 access UNDEF.

+	 */
+	if (vcpu->arch.mdcr_el2 & MDCR_EL2_HPMN_MASK)
+		write_sysreg(0, pmselr_el0);
 	write_sysreg(ARMV8_PMU_USERENR_MASK, pmuserenr_el0);