[PATCH 0/3] support FEAT_MTE_TAGGED_FAR feature

[PATCH 0/3] support FEAT_MTE_TAGGED_FAR feature

[Yeoreum Yun]

The FEAT_MTE_TAGGED_FAR feature provides support for
reporting all non-address bits during a synchronous MTE tag check fault.

This patchset extends the reporting tag to include
not only the memory tag (logical tag) but also the address tag via
si_addr when FEAT_MTE_TAGGED_FAR feature is supported.

Yeoreum Yun (3):
  arm64: add FEAT_MTE_TAGGED_FAR feature
  arm64/mm/fault: use original FAR_EL1 value when ARM64_MTE_FAR is
    supported
  Documentation/arm64: reflects FEAT_MTE_TAGGED_FAR description

 Documentation/arch/arm64/tagged-pointers.rst | 11 ++++++-----
 arch/arm64/include/asm/hwcap.h               |  1 +
 arch/arm64/include/uapi/asm/hwcap.h          |  1 +
 arch/arm64/kernel/cpufeature.c               |  9 +++++++++
 arch/arm64/kernel/cpuinfo.c                  |  1 +
 arch/arm64/mm/fault.c                        |  7 +++++--
 arch/arm64/tools/cpucaps                     |  1 +
 7 files changed, 24 insertions(+), 7 deletions(-)

--
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}

[PATCH 1/3] arm64: add FEAT_MTE_TAGGED_FAR feature

[Yeoreum Yun]

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>
---
 arch/arm64/include/asm/hwcap.h      | 1 +
 arch/arm64/include/uapi/asm/hwcap.h | 1 +
 arch/arm64/kernel/cpufeature.c      | 9 +++++++++
 arch/arm64/kernel/cpuinfo.c         | 1 +
 arch/arm64/tools/cpucaps            | 1 +
 5 files changed, 13 insertions(+)

diff --git a/arch/arm64/include/asm/hwcap.h b/arch/arm64/include/asm/hwcap.h
index 1c3f9617d54f..28dd1ac29ecc 100644
--- a/arch/arm64/include/asm/hwcap.h
+++ b/arch/arm64/include/asm/hwcap.h
@@ -176,6 +176,7 @@
 #define KERNEL_HWCAP_POE		__khwcap2_feature(POE)
 
 #define __khwcap3_feature(x)		(const_ilog2(HWCAP3_ ## x) + 128)
+#define KERNEL_HWCAP_MTE_FAR		__khwcap3_feature(MTE_FAR)
 
 /*
  * This yields a mask that user programs can use to figure out what
diff --git a/arch/arm64/include/uapi/asm/hwcap.h b/arch/arm64/include/uapi/asm/hwcap.h
index 705a7afa8e58..7d22527a7975 100644
--- a/arch/arm64/include/uapi/asm/hwcap.h
+++ b/arch/arm64/include/uapi/asm/hwcap.h
@@ -143,5 +143,6 @@
 /*
  * HWCAP3 flags - for AT_HWCAP3
  */
+#define HWCAP3_MTE_FAR		(1UL << 0)
 
 #endif /* _UAPI__ASM_HWCAP_H */
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 9c4d6d552b25..183b4b7e3074 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -312,6 +312,7 @@ 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,
 };
 
@@ -2861,6 +2862,13 @@ 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)",
@@ -3191,6 +3199,7 @@ static const struct arm64_cpu_capabilities arm64_elf_hwcaps[] = {
 #ifdef CONFIG_ARM64_MTE
 	HWCAP_CAP(ID_AA64PFR1_EL1, MTE, MTE2, CAP_HWCAP, KERNEL_HWCAP_MTE),
 	HWCAP_CAP(ID_AA64PFR1_EL1, MTE, MTE3, CAP_HWCAP, KERNEL_HWCAP_MTE3),
+	HWCAP_CAP(ID_AA64PFR2_EL1, MTEFAR, IMP, CAP_HWCAP, KERNEL_HWCAP_MTE_FAR),
 #endif /* CONFIG_ARM64_MTE */
 	HWCAP_CAP(ID_AA64MMFR0_EL1, ECV, IMP, CAP_HWCAP, KERNEL_HWCAP_ECV),
 	HWCAP_CAP(ID_AA64MMFR1_EL1, AFP, IMP, CAP_HWCAP, KERNEL_HWCAP_AFP),
diff --git a/arch/arm64/kernel/cpuinfo.c b/arch/arm64/kernel/cpuinfo.c
index 285d7d538342..e2b13454e38a 100644
--- a/arch/arm64/kernel/cpuinfo.c
+++ b/arch/arm64/kernel/cpuinfo.c
@@ -160,6 +160,7 @@ static const char *const hwcap_str[] = {
 	[KERNEL_HWCAP_SME_SFEXPA]	= "smesfexpa",
 	[KERNEL_HWCAP_SME_STMOP]	= "smestmop",
 	[KERNEL_HWCAP_SME_SMOP4]	= "smesmop4",
+	[KERNEL_HWCAP_MTE_FAR]		= "mte_far",
 };
 
 #ifdef CONFIG_COMPAT
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index 772c1b008e43..ef62ea04ba37 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -67,6 +67,7 @@ MPAM
 MPAM_HCR
 MTE
 MTE_ASYMM
+MTE_FAR
 SME
 SME_FA64
 SME2
-- 
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Re: [PATCH 1/3] arm64: add FEAT_MTE_TAGGED_FAR feature

[Mark Brown]

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On Thu, Apr 03, 2025 at 03:15:33PM +0100, Yeoreum Yun 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>
> ---
>  arch/arm64/include/asm/hwcap.h      | 1 +
>  arch/arm64/include/uapi/asm/hwcap.h | 1 +
>  arch/arm64/kernel/cpufeature.c      | 9 +++++++++
>  arch/arm64/kernel/cpuinfo.c         | 1 +
>  arch/arm64/tools/cpucaps            | 1 +
>  5 files changed, 13 insertions(+)

Please also add this to the hwcaps kselftest.

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Re: [PATCH 1/3] arm64: add FEAT_MTE_TAGGED_FAR feature

[Yeoreum Yun]

On Thu, Apr 03, 2025 at 04:13:27PM +0100, Mark Brown wrote:
> On Thu, Apr 03, 2025 at 03:15:33PM +0100, Yeoreum Yun 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>
> > ---
> >  arch/arm64/include/asm/hwcap.h      | 1 +
> >  arch/arm64/include/uapi/asm/hwcap.h | 1 +
> >  arch/arm64/kernel/cpufeature.c      | 9 +++++++++
> >  arch/arm64/kernel/cpuinfo.c         | 1 +
> >  arch/arm64/tools/cpucaps            | 1 +
> >  5 files changed, 13 insertions(+)
>
> Please also add this to the hwcaps kselftest.

Thanks. I'll add.

--
Sincerely,
Yeoreum Yun

[PATCH 2/3] arm64/mm/fault: use original FAR_EL1 value when ARM64_MTE_FAR is supported

[Yeoreum Yun]

Use the original FAR_EL1 value when an MTE tag check fault occurs,
if ARM64_MTE_FAR is supported.
This allows reports to include not only the logical tag (memory tag)
but also the address tag information.

Applications that require this information should install a signal handler with
the SA_EXPOSE_TAGBITS flag.
While this introduces a minor ABI change,
most applications do not set this flag and therefore will not be affected.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
---
 arch/arm64/mm/fault.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index ec0a337891dd..8d10603e574f 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -837,9 +837,12 @@ static int do_tag_check_fault(unsigned long far, unsigned long esr,
 	/*
 	 * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN
 	 * for tag check faults. Set them to corresponding bits in the untagged
-	 * address.
+	 * address if ARM64_MTE_FAR doesn't support.
+	 * Otherwise, bits 63:60 of FAR_EL1 are KNOWN.
 	 */
-	far = (__untagged_addr(far) & ~MTE_TAG_MASK) | (far & MTE_TAG_MASK);
+	if (!cpus_have_cap(ARM64_MTE_FAR))
+		far = (__untagged_addr(far) & ~MTE_TAG_MASK) | (far & MTE_TAG_MASK);
+
 	do_bad_area(far, esr, regs);
 	return 0;
 }
-- 
LEVI:{C3F47F37-75D8-414A-A8BA-3980EC8A46D7}

[PATCH 3/3] Documentation/arm64: reflects FEAT_MTE_TAGGED_FAR description

[Yeoreum Yun]

When FEAT_MTE_TAGGED_FAR feature is supported, the value of address bits
[63:60] is preserved on synchronous tag check fault.

This patch reflects the above feature in the documentation.

Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
---
 Documentation/arch/arm64/tagged-pointers.rst | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/Documentation/arch/arm64/tagged-pointers.rst b/Documentation/arch/arm64/tagged-pointers.rst
index 81b6c2a770dd..73c59a9c7a63 100644
--- a/Documentation/arch/arm64/tagged-pointers.rst
+++ b/Documentation/arch/arm64/tagged-pointers.rst
@@ -60,11 +60,12 @@ that signal handlers in applications making use of tags cannot rely
 on the tag information for user virtual addresses being maintained
 in these fields unless the flag was set.
 
-Due to architecture limitations, bits 63:60 of the fault address
-are not preserved in response to synchronous tag check faults
-(SEGV_MTESERR) even if SA_EXPOSE_TAGBITS was set. Applications should
-treat the values of these bits as undefined in order to accommodate
-future architecture revisions which may preserve the bits.
+If FEAT_MTE_TAGGED_FAR (Armv8.9) is supported, bits 63:60 of the fault address
+are preserved in response to synchronous tag check faults (SEGV_MTESERR)
+otherwise not preserved even if SA_EXPOSE_TAGBITS was set.
+Applications should interpret the values of these bits based on
+the support for the 'mte_far' hwcap. If the support is not present,
+the values of these bits should be considered as undefined otherwise valid.
 
 For signals raised in response to watchpoint debug exceptions, the
 tag information will be preserved regardless of the SA_EXPOSE_TAGBITS
-- 
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Re: [PATCH 3/3] Documentation/arm64: reflects FEAT_MTE_TAGGED_FAR description

[Mark Brown]

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On Thu, Apr 03, 2025 at 03:15:35PM +0100, Yeoreum Yun wrote:
> When FEAT_MTE_TAGGED_FAR feature is supported, the value of address bits
> [63:60] is preserved on synchronous tag check fault.
> 
> This patch reflects the above feature in the documentation.
> 
> Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> ---
>  Documentation/arch/arm64/tagged-pointers.rst | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)

I saw the subject line and was looking for the elf_hwcaps.rst update in
this patch, though really it should be in patch 1.

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Re: [PATCH 3/3] Documentation/arm64: reflects FEAT_MTE_TAGGED_FAR description

[Yeoreum Yun]

Hi Mark,

> On Thu, Apr 03, 2025 at 03:15:35PM +0100, Yeoreum Yun wrote:
> > When FEAT_MTE_TAGGED_FAR feature is supported, the value of address bits
> > [63:60] is preserved on synchronous tag check fault.
> >
> > This patch reflects the above feature in the documentation.
> >
> > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > ---
> >  Documentation/arch/arm64/tagged-pointers.rst | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
>
> I saw the subject line and was looking for the elf_hwcaps.rst update in
> this patch, though really it should be in patch 1.

Thanks to let me know. I'll squash it to patch 1.

--
Sincerely,
Yeoreum Yun