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SubjectRe: [PATCH V6 4/6] perf, x86: handle multiple records in PEBS buffer
On Thu, Apr 16, 2015 at 02:53:42PM +0200, Peter Zijlstra wrote:
> When the PEBS interrupt threshold is larger than one record and the
> machine supports multiple PEBS events, the records of these events are
> mixed up and we need to demultiplex them.
>
> Demuxing the records is hard because the hardware is deficient. The
> hardware has two issues that, when combined, create impossible scenarios
> to demux.
>
> The first issue is that the 'status' field of the PEBS record is a copy
> of the GLOBAL_STATUS MSR at PEBS assist time. To see why this is a
> problem let us first describe the regular PEBS cycle:
>
> A) the CTRn value reaches 0:
> - the corresponding bit in GLOBAL_STATUS gets set
> - we start arming the hardware assist
>
> < some unspecified amount of time later --
> this could cover multiple events of interest >
>
> B) the hardware assist is armed, any next event will trigger it
>
> C) a matching event happens:
> - the hardware assist triggers and generates a PEBS record
> this includes a copy of GLOBAL_STATUS at this moment
> - if we auto-reload we (re)set CTRn

Is this actually true? Do we reload here or on A ?

> - we clear the relevant bit in GLOBAL_STATUS


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