linux-kernel - Re: [PATCH] perf/x86/intel/pt: Fix sampling using single range output

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Date:   Mon, 14 Nov 2022 11:51:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] perf/x86/intel/pt: Fix sampling using single range output

On Sat, Nov 12, 2022 at 05:15:08PM +0200, Adrian Hunter wrote:
> Deal with errata TGL052, ADL037 and RPL017 "Trace May Contain Incorrect
> Data When Configured With Single Range Output Larger Than 4KB" by
> disabling single range output whenever larger than 4KB.
> 
> Fixes: 670638477aed ("perf/x86/intel/pt: Opportunistically use single range output mode")
> Cc: stable@...r.kernel.org
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> ---
>  arch/x86/events/intel/pt.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/events/intel/pt.c b/arch/x86/events/intel/pt.c
> index 82ef87e9a897..42a55794004a 100644
> --- a/arch/x86/events/intel/pt.c
> +++ b/arch/x86/events/intel/pt.c
> @@ -1263,6 +1263,15 @@ static int pt_buffer_try_single(struct pt_buffer *buf, int nr_pages)
>  	if (1 << order != nr_pages)
>  		goto out;
>  
> +	/*
> +	 * Some processors cannot always support single range for more than
> +	 * 4KB - refer errata TGL052, ADL037 and RPL017. Future processors might
> +	 * also be affected, so for now rather than trying to keep track of
> +	 * which ones, just disable it for all.
> +	 */
> +	if (nr_pages > 1)
> +		goto out;

This effectively declares single-output-mode dead? Because I don't think
anybody uses PT with a single 4K buffer.