Re: [PATCH v1 1/6] perf: arm64: Add SVE vector granule register to user regs

[Mark Brown <broonie@kernel.org>]

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On Mon, May 09, 2022 at 03:42:49PM +0100, James Clark wrote:
> Dwarf based unwinding in a function that pushes SVE registers onto
> the stack requires the unwinder to know the length of the SVE register
> to calculate the stack offsets correctly. This was added to the Arm
> specific Dwarf spec as the VG pseudo register[1].
> 
> Add the vector length at position 46 if it's requested by userspace and
> SVE is supported. If it's not supported then fail to open the event.
> 
> The vector length must be on each sample because it can be changed
> at runtime via a prctl or ptrace call. Also by adding it as a register
> rather than a separate attribute, minimal changes will be required in an
> unwinder that already indexes into the register list.

> +static u64 perf_ext_regs_value(int idx)
> +{
> +	switch (idx) {
> +	case PERF_REG_ARM64_VG:
> +		if (WARN_ON_ONCE(!system_supports_sve()))
> +			return 0;

These WARN_ON_ONCE()s seem a bit loud but I do see they are idiomatic
for this code so

Reviewed-by: Mark Brown <broonie@kernel.org>

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