linux-kernel - [PATCH 3/3] Documentation/arm64: reflects FEAT_MTE_TAGGED

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Message-Id: <20250403141535.23496-4-yeoreum.yun@arm.com>
Date: Thu,  3 Apr 2025 15:15:35 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: catalin.marinas@....com,
	will@...nel.org,
	broonie@...nel.org,
	anshuman.khandual@....com,
	joey.gouly@....com,
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Cc: linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	Yeoreum Yun <yeoreum.yun@....com>
Subject: [PATCH 3/3] Documentation/arm64: reflects FEAT_MTE_TAGGED_FAR description

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