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

From: Linus Torvalds
Date: Fri Jul 06 2012 - 14:34:57 EST

On Fri, Jul 6, 2012 at 11:16 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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

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

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