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Message-Id: <20250507095757.1663684-3-yeoreum.yun@arm.com>
Date: Wed, 7 May 2025 10:57:53 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: catalin.marinas@....com,
pcc@...gle.com,
will@...nel.org,
broonie@...nel.org,
anshuman.khandual@....com,
joey.gouly@....com,
yury.khrustalev@....com,
maz@...nel.org,
oliver.upton@...ux.dev,
frederic@...nel.org,
shmeerali.kolothum.thodi@...wei.com,
james.morse@....com,
mark.rutland@....com,
huangxiaojia2@...wei.com,
akpm@...ux-foundation.org,
surenb@...gle.com,
robin.murphy@....com
Cc: linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org,
Yeoreum Yun <yeoreum.yun@....com>
Subject: [PATCH v4 2/6] arm64: report address tag when FEAT_MTE_TAGGED_FAR is supported
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).
This patch modifies below to support this feature:
- Use the original FAR_EL1 value when an MTE tag check fault occurs,
if ARM64_MTE_FAR is supported so that not only logical tag
(bits 59:56) but also address tag (bits 63:60] being reported too.
- Add HWCAP for mtefar to let user know bits 63:60 includes
address tag information when when FEAT_MTE_TAGGED_FAR is supported.
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@....com>
---
Documentation/arch/arm64/elf_hwcaps.rst | 3 +++
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 | 1 +
arch/arm64/kernel/cpuinfo.c | 1 +
arch/arm64/mm/fault.c | 7 +++++--
7 files changed, 18 insertions(+), 7 deletions(-)
diff --git a/Documentation/arch/arm64/elf_hwcaps.rst b/Documentation/arch/arm64/elf_hwcaps.rst
index 69d7afe56853..358f5af035ff 100644
--- a/Documentation/arch/arm64/elf_hwcaps.rst
+++ b/Documentation/arch/arm64/elf_hwcaps.rst
@@ -435,6 +435,9 @@ HWCAP2_SME_SF8DP4
HWCAP2_POE
Functionality implied by ID_AA64MMFR3_EL1.S1POE == 0b0001.
+HWCAP3_MTE_FAR
+ Functionality implied by ID_AA64PFR2_EL1.MTEFAR == 0b0001.
+
4. Unused AT_HWCAP bits
-----------------------
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
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 fdb3c1cef236..183b4b7e3074 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -3199,6 +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..058e21295003 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] = "mtefar",
};
#ifdef CONFIG_COMPAT
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index ec0a337891dd..f21d972f99b1 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 isn't supported.
+ * 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}
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