RE: [bug] kpti, perf_event, bts: sporadic truncated trace

[Metzger, Markus T]

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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