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    SubjectRE: [bug] kpti, perf_event, bts: sporadic truncated trace
    Date
    Hello Hugh,

    > A little "optimization" crept into alloc_bts_buffer() along the way, which now
    > places bts_interrupt_threshold not on a record boundary.
    > And Stephane has shown me the sentence in Vol 3B, 17.4.9, which says "This
    > address must point to an offset from the BTS buffer base that is a multiple of the
    > BTS record size."
    >
    > Please give the patch below a try, and let us know if it helps (if it does not, then I
    > think we'll need perfier expertise than I can give).
    >
    > Hugh
    >
    > --- 4.18-rc4/arch/x86/events/intel/ds.c 2018-06-03 14:15:21.000000000 -0700
    > +++ linux/arch/x86/events/intel/ds.c 2018-07-12 17:38:28.471378616 -0700
    > @@ -408,9 +408,11 @@ static int alloc_bts_buffer(int cpu)
    > ds->bts_buffer_base = (unsigned long) cea;
    > ds_update_cea(cea, buffer, BTS_BUFFER_SIZE, PAGE_KERNEL);
    > ds->bts_index = ds->bts_buffer_base;
    > - max = BTS_RECORD_SIZE * (BTS_BUFFER_SIZE / BTS_RECORD_SIZE);
    > - ds->bts_absolute_maximum = ds->bts_buffer_base + max;
    > - ds->bts_interrupt_threshold = ds->bts_absolute_maximum - (max / 16);
    > + max = BTS_BUFFER_SIZE / BTS_RECORD_SIZE;
    > + ds->bts_absolute_maximum = ds->bts_buffer_base +
    > + max * BTS_RECORD_SIZE;
    > + ds->bts_interrupt_threshold = ds->bts_absolute_maximum -
    > + (max / 16) * BTS_RECORD_SIZE;
    > return 0;
    > }
    >

    Your patch fixes it.

    Will you send it to the list for inclusion? I'd appreciate if it could also be backported
    to 4.15, 4.16, and 4.17.

    Thanks,
    Markus.

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