Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
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- Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
- From: Ingo Molnar <mingo@xxxxxxx>
- Date: Mon, 15 Jun 2009 23:16:05 +0200
- Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, mingo@xxxxxxxxxx, paulus@xxxxxxxxx, acme@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, penberg@xxxxxxxxxxxxxx, vegard.nossum@xxxxxxxxx, efault@xxxxxx, jeremy@xxxxxxxx, npiggin@xxxxxxx, tglx@xxxxxxxxxxxxx, linux-tip-commits@xxxxxxxxxxxxxxx
- In-reply-to: <20090615211207.GB12919@Krystal>
- References: <4A369508.2090707@xxxxxxxxx> <20090615184858.GD6520@Krystal> <1245091917.6741.185.camel@laptop> <20090615185907.GF6520@Krystal> <1245092561.6741.205.camel@laptop> <4A369CD8.3090505@xxxxxxxxx> <20090615192720.GA9056@Krystal> <4A36A1C7.6080005@xxxxxxxxx> <20090615210119.GD24554@xxxxxxx> <20090615211207.GB12919@Krystal>
- User-agent: Mutt/1.5.18 (2008-05-17)
* Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:
> Just for the sake of making NMI handlers less tricky, supporting
> page faults caused by faulting kernel instructions (rather than
> only supporting explicit faulting from get_user_pages_inatomic)
> would be rather nice design-wise if it only costs 2-3 cycles.
>
> And I would not want to touch the page fault handler itself to
> write the saved cr2 value before the handler exits, because this
> would add a branch on a very hot path.
_That_ path is not hot at all - it's the 'we are in atomic section
and faulted' rare path (laced with an exception table search - which
is extremely slow compared to other bits of the pagefault path).
But ... it's not an issue: a check can be made in the NMI code too,
as we always know about pagefaults there, by virtue of getting
-EFAULT back from the attempted-user-copy.
Ingo
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