Implement the GCIE_LEGACY capability as a system feature to be able to
check for support from KVM. The type is explicitly
ARM64_CPUCAP_EARLY_LOCAL_CPU_FEATURE, which means that the capability
is enabled early if all boot CPUs support it. Additionally, if this
capability is enabled during boot, it prevents late onlining of CPUs
that lack it, thereby avoiding potential mismatched configurations
which would break KVM.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
---
 arch/arm64/kernel/cpufeature.c | 15 +++++++++++++++
 arch/arm64/tools/cpucaps       |  1 +
 2 files changed, 16 insertions(+)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9ad065f15f1d..afb3b10afd75 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2520,6 +2520,15 @@ test_has_mpam_hcr(const struct arm64_cpu_capabilities *entry, int scope)
 	return idr & MPAMIDR_EL1_HAS_HCR;
 }
 
+static bool
+test_has_gicv5_legacy(const struct arm64_cpu_capabilities *entry, int scope)
+{
+	if (!this_cpu_has_cap(ARM64_HAS_GICV5_CPUIF))
+		return false;
+
+	return !!(read_sysreg_s(SYS_ICC_IDR0_EL1) & ICC_IDR0_EL1_GCIE_LEGACY);
+}
+
 static const struct arm64_cpu_capabilities arm64_features[] = {
 	{
 		.capability = ARM64_ALWAYS_BOOT,
@@ -3131,6 +3140,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.matches = has_cpuid_feature,
 		ARM64_CPUID_FIELDS(ID_AA64PFR2_EL1, GCIE, IMP)
 	},
+	{
+		.desc = "GICv5 Legacy vCPU interface",
+		.type = ARM64_CPUCAP_EARLY_LOCAL_CPU_FEATURE,
+		.capability = ARM64_HAS_GICV5_LEGACY,
+		.matches = test_has_gicv5_legacy,
+	},
 	{},
 };
 
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index ef0b7946f5a4..d055664613e6 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -37,6 +37,7 @@ HAS_GENERIC_AUTH_ARCH_QARMA5
 HAS_GENERIC_AUTH_IMP_DEF
 HAS_GICV3_CPUIF
 HAS_GICV5_CPUIF
+HAS_GICV5_LEGACY
 HAS_GIC_PRIO_MASKING
 HAS_GIC_PRIO_RELAXED_SYNC
 HAS_HCR_NV1
-- 
2.34.1

On 28/08/2025 11:59, Sascha Bischoff wrote:
> Implement the GCIE_LEGACY capability as a system feature to be able to
> check for support from KVM. The type is explicitly
> ARM64_CPUCAP_EARLY_LOCAL_CPU_FEATURE, which means that the capability
> is enabled early if all boot CPUs support it. Additionally, if this
> capability is enabled during boot, it prevents late onlining of CPUs
> that lack it, thereby avoiding potential mismatched configurations
> which would break KVM.
> 
> Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>


> ---
>   arch/arm64/kernel/cpufeature.c | 15 +++++++++++++++
>   arch/arm64/tools/cpucaps       |  1 +
>   2 files changed, 16 insertions(+)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9ad065f15f1d..afb3b10afd75 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -2520,6 +2520,15 @@ test_has_mpam_hcr(const struct arm64_cpu_capabilities *entry, int scope)
>   	return idr & MPAMIDR_EL1_HAS_HCR;
>   }
>   
> +static bool
> +test_has_gicv5_legacy(const struct arm64_cpu_capabilities *entry, int scope)
> +{
> +	if (!this_cpu_has_cap(ARM64_HAS_GICV5_CPUIF))
> +		return false;
> +
> +	return !!(read_sysreg_s(SYS_ICC_IDR0_EL1) & ICC_IDR0_EL1_GCIE_LEGACY);
> +}
> +
>   static const struct arm64_cpu_capabilities arm64_features[] = {
>   	{
>   		.capability = ARM64_ALWAYS_BOOT,
> @@ -3131,6 +3140,12 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>   		.matches = has_cpuid_feature,
>   		ARM64_CPUID_FIELDS(ID_AA64PFR2_EL1, GCIE, IMP)
>   	},
> +	{
> +		.desc = "GICv5 Legacy vCPU interface",
> +		.type = ARM64_CPUCAP_EARLY_LOCAL_CPU_FEATURE,

This is the right type for the capability intended, running the test on 
each boot time CPUs and setting the cap accordingly.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>