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arm64/sve: Add Perf extensions documentation

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Document that the VG register is available in Perf samples

Signed-off-by: James Clark <james.clark@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220901132658.1024635-3-james.clark@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
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James Clark 2022-09-01 14:26:58 +01:00 committed by Will Deacon
parent cbb0c02caf
commit 1f2906d1e1
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Documentation/arm64/sve.rst
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@ -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