[PATCH] KVM: arm64: Treat ID_AA64DFR0_EL1.PMUVer as an unsigned field

[PATCH] KVM: arm64: Treat ID_AA64DFR0_EL1.PMUVer as an unsigned field

[Jing Zhang]

ID_AA64DFR0_EL1.PMUVer is an unsigned 4-bit field, but was historically
marked as signed in the cpufeature table. This led
kvm_init_host_debug_data() to fail detection of PMUv3 implementation on
systems with PMUv3p8 or newer, as the signed extraction of the field
(e.g., 0b1000 for PMUv3p8) would result in a negative value.

Fix this by marking the field as unsigned in the ftr_id_aa64dfr0 table
and updating the KVM initialization code to use unsigned extraction.
While at it, ensure that both the 'Not Implemented' (0b0000) and
'Implementation Defined' (0b1111) values are correctly handled as
indicating the absence of a standard PMUv3.

Signed-off-by: Jing Zhang <jingzhangos@google.com>
---
 arch/arm64/kernel/cpufeature.c | 2 +-
 arch/arm64/kvm/debug.c         | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 32c2dbcc0c641..1b5e37a1a41b4 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -565,7 +565,7 @@ static const struct arm64_ftr_bits ftr_id_aa64dfr0[] = {
 	 * We can instantiate multiple PMU instances with different levels
 	 * of support.
 	 */
-	S_ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_EXACT, ID_AA64DFR0_EL1_PMUVer_SHIFT, 4, 0),
+	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_EXACT, ID_AA64DFR0_EL1_PMUVer_SHIFT, 4, 0),
 	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_EXACT, ID_AA64DFR0_EL1_DebugVer_SHIFT, 4, 0x6),
 	ARM64_FTR_END,
 };
diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index 3ad6b7c6e4ba7..2834d7ef9dedb 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c
@@ -75,8 +75,10 @@ static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu)
 void kvm_init_host_debug_data(void)
 {
 	u64 dfr0 = read_sysreg(id_aa64dfr0_el1);
+	u64 pmuver;
 
-	if (cpuid_feature_extract_signed_field(dfr0, ID_AA64DFR0_EL1_PMUVer_SHIFT) > 0)
+	pmuver = cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_PMUVer_SHIFT);
+	if (pmuv3_implemented(pmuver))
 		*host_data_ptr(nr_event_counters) = FIELD_GET(ARMV8_PMU_PMCR_N,
 							      read_sysreg(pmcr_el0));
 
-- 
2.54.0.rc1.555.g9c883467ad-goog

Re: [PATCH] KVM: arm64: Treat ID_AA64DFR0_EL1.PMUVer as an unsigned field

[Marc Zyngier]

On Tue, 21 Apr 2026 17:41:12 +0100,
Jing Zhang <jingzhangos@google.com> wrote:
> 
> ID_AA64DFR0_EL1.PMUVer is an unsigned 4-bit field, but was historically
> marked as signed in the cpufeature table. This led
> kvm_init_host_debug_data() to fail detection of PMUv3 implementation on
> systems with PMUv3p8 or newer, as the signed extraction of the field
> (e.g., 0b1000 for PMUv3p8) would result in a negative value.
> 
> Fix this by marking the field as unsigned in the ftr_id_aa64dfr0 table
> and updating the KVM initialization code to use unsigned extraction.
> While at it, ensure that both the 'Not Implemented' (0b0000) and
> 'Implementation Defined' (0b1111) values are correctly handled as
> indicating the absence of a standard PMUv3.
> 
> Signed-off-by: Jing Zhang <jingzhangos@google.com>

https://lore.kernel.org/all/20260305-james-kvm-pmuver-sign-v2-0-ee80a125af9b@linaro.org/

	M.

-- 
Without deviation from the norm, progress is not possible.