linux-kernel - Re: [PATCH 31/35] perf intel-pt: Disable sync switch with guest sideband

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Date:   Tue, 19 Jul 2022 18:14:40 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 31/35] perf intel-pt: Disable sync switch with guest sideband

On Mon, Jul 11, 2022 at 2:33 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
>
> The sync_switch facility attempts to better synchronize context switches
> with the Intel PT trace, however it is not designed for guest machine
> context switches, so disable it when guest sideband is detected.
>
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>

Acked-by: Ian Rogers <irogers@...gle.com>

Thanks,
Ian

> ---
>  tools/perf/util/intel-pt.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
>
> diff --git a/tools/perf/util/intel-pt.c b/tools/perf/util/intel-pt.c
> index 98b097fec476..dc2af64f9e31 100644
> --- a/tools/perf/util/intel-pt.c
> +++ b/tools/perf/util/intel-pt.c
> @@ -74,6 +74,7 @@ struct intel_pt {
>         bool data_queued;
>         bool est_tsc;
>         bool sync_switch;
> +       bool sync_switch_not_supported;
>         bool mispred_all;
>         bool use_thread_stack;
>         bool callstack;
> @@ -2638,6 +2639,9 @@ static void intel_pt_enable_sync_switch(struct intel_pt *pt)
>  {
>         unsigned int i;
>
> +       if (pt->sync_switch_not_supported)
> +               return;
> +
>         pt->sync_switch = true;
>
>         for (i = 0; i < pt->queues.nr_queues; i++) {
> @@ -2649,6 +2653,23 @@ static void intel_pt_enable_sync_switch(struct intel_pt *pt)
>         }
>  }
>
> +static void intel_pt_disable_sync_switch(struct intel_pt *pt)
> +{
> +       unsigned int i;
> +
> +       pt->sync_switch = false;
> +
> +       for (i = 0; i < pt->queues.nr_queues; i++) {
> +               struct auxtrace_queue *queue = &pt->queues.queue_array[i];
> +               struct intel_pt_queue *ptq = queue->priv;
> +
> +               if (ptq) {
> +                       ptq->sync_switch = false;
> +                       intel_pt_next_tid(pt, ptq);
> +               }
> +       }
> +}
> +
>  /*
>   * To filter against time ranges, it is only necessary to look at the next start
>   * or end time.
> @@ -3090,6 +3111,14 @@ static int intel_pt_guest_context_switch(struct intel_pt *pt,
>
>         pt->have_guest_sideband = true;
>
> +       /*
> +        * sync_switch cannot handle guest machines at present, so just disable
> +        * it.
> +        */
> +       pt->sync_switch_not_supported = true;
> +       if (pt->sync_switch)
> +               intel_pt_disable_sync_switch(pt);
> +
>         if (out)
>                 return 0;
>
> --
> 2.25.1
>