linux-kernel - Re: [tip:perf/core] perf/x86: Fix USER/KERNEL tagging of samples

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Date:	Fri, 6 Jul 2012 11:34:35 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	mingo@...nel.org, hpa@...or.com, eranian@...gle.com,
	linux-kernel@...r.kernel.org, fweisbec@...il.com,
	akpm@...ux-foundation.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf/x86: Fix USER/KERNEL tagging of samples

On Fri, Jul 6, 2012 at 11:16 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> If some code then changes the values in the pt_regs, it is *that* code
> that needs to think twice about what it does. Where is that code?

>From a quick grep it looks like it is __intel_pmu_pebs_event() that does this.

THAT is where you would possibly have a huge honking big comment about
how you have to fake the CS register contents because the PEBS
information is incomplete. But make it clear that it is a total hack.

Also, somebody should check. Is the PEBS information *actually* the
instruction pointer (address within the code segment), or is it the
"linear address" (segment base + rip)? I hope it is the latter,
because in the absense of CS, the segment-based address is very
unclear indeed.

And if it *is* the linear address, then at that point you could do

   regs->cs = kernel_ip(ip) ? __KERNEL_CS : __USER_CS;
   regs->eflags &= ~X86_EFLAGS_VM;

and document this as a "we fake the CS and vm86 mode, using the known
zero-based code segments". At that point it would be technically
correct.

But any code that does "kernel_ip(regs->ip)" is just terminally
confused and can never be sane.

                  Linus
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