Re: [PATCH] perf/x86/amd/lbr: Freeze LBR on PMI only when kernel branches are recorded

[Ravi Bangoria <ravi.bangoria@amd.com>]

On 23-Oct-26 6:11 PM, Puranjay Mohan wrote:
> On CPUs with X86_FEATURE_AMD_LBR_PMC_FREEZE the LBR is always frozen on
> PMI, so the branch stack ends before the IP and callchain, which are
> taken when the NMI is delivered. IBS rejects branch-stack events, so
> there is no precise alternative.
> 
> The freeze only keeps the PMI handler's branches out of the stack, which
> matters only when branches into the kernel are recorded; without it the
> stack fills with the handler's own branches. A user-only filter does not
> record them, and parts without the feature are never frozen (see commit
> 598c2fafc06f ("perf/x86/amd/lbr: Use freeze based on availability")).
> 
> Freeze unless the value written to LBR_SELECT, which is shared by all
> events on the CPU, suppresses branches into the kernel.
> 
> With perf record -e ex_ret_brn_tkn:u -j any,u on a Zen 5 part running
> CPython, the function of the sampled IP differs from the function of
> LBR[0].to in 84.3% of samples with the freeze and in 0.1% without it.
> 
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

Reviewed-by: Ravi Bangoria <ravi.bangoria@amd.com>

Thanks,
Ravi