Re: [PATCH] perf/x86/amd: Reject AMD Merge event as a countable event

[Sandipan Das <sandipan.das@amd.com>]

On 16-09-2026 18:03, Luka Absandze wrote:
> AMD event select 0xFFF (Merge) is the hardware control encoding used by
> Large Increment per Cycle (LIPC) pairs.  When written into an odd
> counter's PerfCtl it turns that counter into the upper 16 bits of a
> 64-bit merged accumulator; any subsequent arming write (-(2^47-1)) is
> silently truncated to its low 16 bits, while hwc->prev_count retains the
> full value.
> 
> This is particularly problematic with a KVM guest, which will program
> the Merge encoding into its own odd vPMC as a normal part of scheduling
> a LIPC event.  KVM intercepts that write and faithfully forwards it to
> the host as a raw event carrying the Merge encoding, where it is
> accepted as an ordinary countable event. Once a counter's PerfCtl holds
> the Merge encoding, every later user of that counter gets a first delta
> inflated by 2^47
> 
> Reject Merge (AMD_MERGE_EVENT) in amd_core_hw_config() so it cannot be
> submitted as a user-visible event.  The pair mechanism programs it
> itself via x86_pmu.perf_ctr_pair_en; nothing else should do so.
> 
> Fixes: 5738891229a2 ("perf/x86/amd: Add support for Large Increment per Cycle Events")
> Reported-by: Yao Zhou <yaoz@meta.com>
> Signed-off-by: Luka Absandze <absandze@amazon.de>
> ---
>  arch/x86/events/amd/core.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> index 49b6b8fce566..cb655c751e5b 100644
> --- a/arch/x86/events/amd/core.c
> +++ b/arch/x86/events/amd/core.c
> @@ -380,8 +380,12 @@ static int amd_core_hw_config(struct perf_event *event)
>  	else if (event->attr.exclude_guest)
>  		event->hw.config |= AMD64_EVENTSEL_HOSTONLY;
>  
> -	if ((x86_pmu.flags & PMU_FL_PAIR) && amd_is_pair_event_code(&event->hw))
> -		event->hw.flags |= PERF_X86_EVENT_PAIR;
> +	if (x86_pmu.flags & PMU_FL_PAIR) {
> +		if ((event->hw.config & AMD64_EVENTSEL_EVENT) == AMD_MERGE_EVENT)
> +			return -EINVAL;
> +		if (amd_is_pair_event_code(&event->hw))
> +			event->hw.flags |= PERF_X86_EVENT_PAIR;
> +	}
>  
>  	if (has_branch_stack(event))
>  		return static_call(amd_pmu_branch_hw_config)(event);

Reviewed-by: Sandipan Das <sandipan.das@amd.com>