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
> 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
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.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/