[PATCH v3 2/2] arm64/sve: Add Perf extensions documentation

James Clark
2022-09-01T13:36:40Z

Document that the VG register is available in Perf samples

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

diff --git a/Documentation/arm64/sve.rst b/Documentation/arm64/sve.rst
index 93c2c2990584..8955bf1bf757 100644
--- a/Documentation/arm64/sve.rst
+++ b/Documentation/arm64/sve.rst
@@ -452,6 +452,24 @@ 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)
=================================================
@@ -593,3 +611,5 @@ 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