* Re: [PATCH] KVM: arm64: Check ID_AA64DFR0_EL1.PMUVer in reset_pmevtyper()
2026-09-29 22:04 [PATCH] KVM: arm64: Check ID_AA64DFR0_EL1.PMUVer in reset_pmevtyper() Colton Lewis
@ 2026-09-29 22:59 ` Oliver Upton
0 siblings, 0 replies; 2+ messages in thread
From: Oliver Upton @ 2026-09-29 22:59 UTC (permalink / raw)
To: Colton Lewis
Cc: kvmarm, Marc Zyngier, Fuad Tabba, Joey Gouly, Steffen Eiden,
Suzuki K Poulose, Zenghui Yu, Catalin Marinas, Will Deacon,
Mark Rutland, linux-arm-kernel, linux-kernel, stable
Hi Colton,
On Tue, Sep 29, 2026 at 10:04:11PM +0000, Colton Lewis wrote:
> When userspace initializes a vCPU with KVM_ARM_VCPU_PMU_V3 but sets
> ID_AA64DFR0_EL1.PMUVer to NI (0) or IMP_DEF (0xf, which is sanitized to
> NI), resetting the vCPU invokes reset_pmevtyper(). Because
> reset_pmevtyper() only checks kvm_vcpu_has_pmu(), it proceeds to call
> kvm_pmu_evtyper_mask(), which reads PMUVer == 0 from ID_AA64DFR0_EL1 and
> triggers a WARN_ONCE("Unknown PMU version 0") in __kvm_pmu_event_mask().
>
> Check ID_AA64DFR0_EL1.PMUVer directly in reset_pmevtyper() via
> kvm_has_feat() instead of kvm_vcpu_has_pmu() so that PMEVTYPER<n>_EL0
> resets to 0 without querying the event mask when PMUv3 is not exposed to
> the guest.
So this goes a bit further than just this WARN_ONCE()... Ultimately if
FEAT_PMUv3 is hidden via the ID registers then we should expect KVM to
emulate these sysregs as UNDEF.
> Fixes: bc512d6a9b92 ("KVM: arm64: Make PMEVTYPER<n>_EL0.NSH RES0 if EL2 isn't advertised")
> Suggested-by: Oliver Upton <oupton@kernel.org>
> Cc: stable@vger.kernel.org
Avoiding a WARN_ONCE() is not stable material.
> Assisted-by: LLM
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
> arch/arm64/kvm/sys_regs.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index a2f4e769a4282..0bb3c2f10a22c 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1060,7 +1060,7 @@ static u64 reset_pmevcntr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> static u64 reset_pmevtyper(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
> {
> /* This thing will UNDEF, who cares about the reset value? */
> - if (!kvm_vcpu_has_pmu(vcpu))
> + if (!kvm_has_feat(vcpu->kvm, ID_AA64DFR0_EL1, PMUVer, IMP))
> return 0;
I have this diff locally, separating the visibility change into a
separate patch. Do you want to give it a spin?
Thanks,
Oliver
diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
index 5b1af7e2176f..e2b28dbc0451 100644
--- a/arch/arm64/kvm/pmu-emul.c
+++ b/arch/arm64/kvm/pmu-emul.c
@@ -50,8 +50,11 @@ static u32 __kvm_pmu_event_mask(unsigned int pmuver)
case ID_AA64DFR0_EL1_PMUVer_V3P5:
case ID_AA64DFR0_EL1_PMUVer_V3P7:
return GENMASK(15, 0);
- default: /* Shouldn't be here, just for sanity */
+ default:
+ /* Shouldn't be here, just for sanity */
WARN_ONCE(1, "Unknown PMU version %d\n", pmuver);
+ fallthrough;
+ case ID_AA64DFR0_EL1_PMUVer_NI:
return 0;
}
}
diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 44aae52c473d..8315a188676f 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1028,7 +1028,7 @@ static unsigned int hidden_visibility(const struct kvm_vcpu *vcpu,
static unsigned int pmu_visibility(const struct kvm_vcpu *vcpu,
const struct sys_reg_desc *r)
{
- if (kvm_vcpu_has_pmu(vcpu))
+ if (kvm_has_feat(vcpu->kvm, ID_AA64DFR0_EL1, PMUVer, IMP))
return 0;
return REG_HIDDEN;
@@ -1058,10 +1058,6 @@ static u64 reset_pmevcntr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
static u64 reset_pmevtyper(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r)
{
- /* This thing will UNDEF, who cares about the reset value? */
- if (!kvm_vcpu_has_pmu(vcpu))
- return 0;
-
reset_unknown(vcpu, r);
__vcpu_rmw_sys_reg(vcpu, r->reg, &=, kvm_pmu_evtyper_mask(vcpu->kvm));
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