Re: [Patch v5 02/18] perf/x86/intel: Validate LBR XSAVE state before updating LBR stack

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Oct 29, 2026 at 12:38:58PM +0800, Dapeng Mi wrote:
> intel_pmu_arch_lbr_read_xsave() doesn't check if the LBR data is present
> in the xsave area. If LBR data is in the INIT state, the LBR data may not
> be really written into the xsave area.
> 
> Add an explicit LBR XSAVE state presence check before updating the cached
> LBR stack. Also clear the cached LBR stack when intel_pmu_lbr_reset() is
> called, in addition to resetting the hardware LBR register file, to prevent
> stale LBR data from being reused after a context switch or similar
> transition.
> 
> Fixes: c085fb877467 ("perf/x86/intel/lbr: Support XSAVES for arch LBR read")
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---
>  arch/x86/events/intel/lbr.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> index 0b446d8912e4..a2e4ba7e1a4f 100644
> --- a/arch/x86/events/intel/lbr.c
> +++ b/arch/x86/events/intel/lbr.c
> @@ -193,6 +193,8 @@ void intel_pmu_lbr_reset(void)
>  {
>  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>  
> +	cpuc->lbr_stack.nr = 0;
> +
>  	if (!x86_pmu.lbr_nr)
>  		return;
>  

This also clears the stack if there is no LBR; was that on purpose? The
Changelog is silent on this point.

> @@ -1010,9 +1012,15 @@ static void intel_pmu_arch_lbr_read_xsave(struct cpu_hw_events *cpuc)
>  		intel_pmu_store_lbr(cpuc, NULL);
>  		return;
>  	}
> +	xsave->header.xfeatures = 0;
>  	xsaves(&xsave->xsave, XFEATURE_MASK_LBR);
>  
> -	intel_pmu_store_lbr(cpuc, xsave->lbr.entries);
> +	if (xsave->header.xfeatures & XFEATURE_MASK_LBR) {
> +		intel_pmu_store_lbr(cpuc, xsave->lbr.entries);
> +	} else {
> +		/* Clear the LBR stack to avoid leaking stale entries. */
> +		cpuc->lbr_stack.nr = 0;
> +	}
>  }
>  
>  void intel_pmu_lbr_read(void)
> -- 
> 2.34.1
>