Add FEAT_MTE_TAGGED_FAR cpucap which makes FAR_ELx report
all non-address bits on a synchronous MTE tag check fault since Armv8.9

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
---
 arch/arm64/kernel/cpufeature.c | 8 ++++++++
 arch/arm64/tools/cpucaps       | 1 +
 2 files changed, 9 insertions(+)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index b34044e20128..af6a6924a3e8 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -320,6 +320,7 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
 
 static const struct arm64_ftr_bits ftr_id_aa64pfr2[] = {
 	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_FPMR_SHIFT, 4, 0),
+	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_MTEFAR_SHIFT, 4, ID_AA64PFR2_EL1_MTEFAR_NI),
 	ARM64_FTR_END,
 };
 
@@ -2874,6 +2875,13 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
 		.matches = has_cpuid_feature,
 		ARM64_CPUID_FIELDS(ID_AA64PFR1_EL1, MTE, MTE3)
 	},
+	{
+		.desc = "FAR on MTE Tag Check Fault",
+		.capability = ARM64_MTE_FAR,
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.matches = has_cpuid_feature,
+		ARM64_CPUID_FIELDS(ID_AA64PFR2_EL1, MTEFAR, IMP)
+	},
 #endif /* CONFIG_ARM64_MTE */
 	{
 		.desc = "RCpc load-acquire (LDAPR)",
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index 10effd4cff6b..fe8f4f8ce95c 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -68,6 +68,7 @@ MPAM
 MPAM_HCR
 MTE
 MTE_ASYMM
+MTE_FAR
 SME
 SME_FA64
 SME2
-- 
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}

On Wed, 11 Jun 2025 14:58:10 +0100,
Yeoreum Yun <yeoreum.yun@arm.com> wrote:
> 
> Add FEAT_MTE_TAGGED_FAR cpucap which makes FAR_ELx report
> all non-address bits on a synchronous MTE tag check fault since Armv8.9
> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
> ---
>  arch/arm64/kernel/cpufeature.c | 8 ++++++++
>  arch/arm64/tools/cpucaps       | 1 +
>  2 files changed, 9 insertions(+)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index b34044e20128..af6a6924a3e8 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -320,6 +320,7 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
>  
>  static const struct arm64_ftr_bits ftr_id_aa64pfr2[] = {
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_FPMR_SHIFT, 4, 0),
> +	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_MTEFAR_SHIFT, 4, ID_AA64PFR2_EL1_MTEFAR_NI),
>  	ARM64_FTR_END,
>  };

Same thing as the remark I had on FEAT_LSUI: for anything that does
not introduce extra state, please similarly enable the feature in KVM.
There is no point in these discrepancies.

For anything that has extra state, you should at least consider what
needs to be done.

Thanks,

	M.

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

Hi Marc,

> On Wed, 11 Jun 2025 14:58:10 +0100,
> Yeoreum Yun <yeoreum.yun@arm.com> wrote:
> >
> > Add FEAT_MTE_TAGGED_FAR cpucap which makes FAR_ELx report
> > all non-address bits on a synchronous MTE tag check fault since Armv8.9
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
> > ---
> >  arch/arm64/kernel/cpufeature.c | 8 ++++++++
> >  arch/arm64/tools/cpucaps       | 1 +
> >  2 files changed, 9 insertions(+)
> >
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index b34044e20128..af6a6924a3e8 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -320,6 +320,7 @@ static const struct arm64_ftr_bits ftr_id_aa64pfr1[] = {
> >
> >  static const struct arm64_ftr_bits ftr_id_aa64pfr2[] = {
> >  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_FPMR_SHIFT, 4, 0),
> > +	ARM64_FTR_BITS(FTR_VISIBLE, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR2_EL1_MTEFAR_SHIFT, 4, ID_AA64PFR2_EL1_MTEFAR_NI),
> >  	ARM64_FTR_END,
> >  };
>
> Same thing as the remark I had on FEAT_LSUI: for anything that does
> not introduce extra state, please similarly enable the feature in KVM.
> There is no point in these discrepancies.
>
> For anything that has extra state, you should at least consider what
> needs to be done.
>
> Thanks,

Okay. I'll expose this to guest.
Thanks.

--
Sincerely,
Yeoreum Yun