linux-kernel - [PATCH v2 2/2] arm64/sve: Add Perf extensions documentation

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Message-Id: <20220517100743.3020667-3-james.clark@arm.com>
Date:   Tue, 17 May 2022 11:07:43 +0100
From:   James Clark <james.clark@....com>
To:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, broonie@...nel.org,
        acme@...nel.org
Cc:     german.gomez@....com, leo.yan@...aro.org,
        mathieu.poirier@...aro.org, john.garry@...wei.com,
        James Clark <james.clark@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Mark Rutland <mark.rutland@....com>, linux-doc@...r.kernel.org
Subject: [PATCH v2 2/2] arm64/sve: Add Perf extensions documentation

Document that the VG register is available in Perf samples

Signed-off-by: James Clark <james.clark@....com>
---
 Documentation/arm64/sve.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/arm64/sve.rst b/Documentation/arm64/sve.rst
index 9d9a4de5bc34..b34a1467057f 100644
--- a/Documentation/arm64/sve.rst
+++ b/Documentation/arm64/sve.rst
@@ -402,6 +402,24 @@ The regset data starts with struct user_sve_header, containing:
 * Modifying the system default vector length does not affect the vector length
   of any existing process or thread that does not make an execve() call.
 
+10.  Perf extensions
+--------------------------------
+
+* The arm64 specific DWARF standard [5] added the VG (Vector Granule) register
+  at index 46. This register is used for DWARF unwinding when variable length
+  SVE registers are pushed onto the stack.
+
+* Its value is equivalent to the current SVE vector length (VL) in bits divided
+  by 64.
+
+* The value is included in Perf samples in the regs[46] field if
+  PERF_SAMPLE_REGS_USER is set and the sample_regs_user mask has bit 46 set.
+
+* The value is the current value at the time the sample was taken, and it can
+  change over time.
+
+* If the system doesn't support SVE when perf_event_open is called with these
+  settings, the event will fail to open.
 
 Appendix A.  SVE programmer's model (informative)
 =================================================
@@ -543,3 +561,5 @@ References
     http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055c/IHI0055C_beta_aapcs64.pdf
     http://infocenter.arm.com/help/topic/com.arm.doc.subset.swdev.abi/index.html
     Procedure Call Standard for the ARM 64-bit Architecture (AArch64)
+
+[5] https://github.com/ARM-software/abi-aa/blob/main/aadwarf64/aadwarf64.rst
-- 
2.28.0