[PATCH v3 3/3] KVM: arm64: Disable host PMU counters for protected guests

[Fuad Tabba <fuad.tabba@linux.dev>]

A protected guest has no PMU, and EL2 clears MDCR_EL2.HPME for it.
However, __pkvm_vcpu_load() sets HPMN to cover every event counter, so
HPME has no effect and the counters the host has enabled keep counting
the guest.

Clear PMCR_EL0.E in the world switch while a protected guest runs, which
stops them all. Gate this on system_supports_pmuv3(), as
__activate_traps_common() does for PMUSERENR_EL0.

Side channels are outside pKVM's threat model, so this is hardening
rather than a fix.

Reported-by: Oculytic <oculytic@gmail.com>
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 arch/arm64/kvm/hyp/nvhe/switch.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 91e1edf40bcd2..003469e85ed46 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -175,6 +175,26 @@ static void __pmu_switch_to_host(struct kvm_vcpu *vcpu)
 #define __pmu_switch_to_host(v)		do {} while (0)
 #endif
 
+static u64 __pmu_stop_host_counters(struct kvm_vcpu *vcpu)
+{
+	u64 pmcr;
+
+	if (!vcpu_is_protected(vcpu) || !system_supports_pmuv3())
+		return 0;
+
+	pmcr = read_sysreg(pmcr_el0);
+	if (pmcr & ARMV8_PMU_PMCR_E)
+		write_sysreg(pmcr & ~ARMV8_PMU_PMCR_E, pmcr_el0);
+
+	return pmcr;
+}
+
+static void __pmu_restore_host_counters(u64 pmcr)
+{
+	if (pmcr & ARMV8_PMU_PMCR_E)
+		write_sysreg(pmcr, pmcr_el0);
+}
+
 /*
  * Handler for protected VM MSR, MRS or System instruction execution in AArch64.
  *
@@ -283,6 +303,7 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
 	struct kvm_s2_mmu *mmu;
 	bool pmu_switch_needed;
 	u64 exit_code;
+	u64 host_pmcr;
 
 	/*
 	 * Having IRQs masked via PMR when entering the guest means the GIC
@@ -300,6 +321,7 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
 	guest_ctxt = &vcpu->arch.ctxt;
 
 	pmu_switch_needed = __pmu_switch_to_guest(vcpu);
+	host_pmcr = __pmu_stop_host_counters(vcpu);
 
 	__sysreg_save_state_nvhe(host_ctxt);
 	/*
@@ -384,6 +406,8 @@ int __kvm_vcpu_run(struct kvm_vcpu *vcpu)
 	if (pmu_switch_needed)
 		__pmu_switch_to_host(vcpu);
 
+	__pmu_restore_host_counters(host_pmcr);
+
 	/* Returning to host will clear PSR.I, remask PMR if needed */
 	if (system_uses_irq_prio_masking())
 		gic_write_pmr(GIC_PRIO_IRQOFF);
-- 
2.47.3