Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 6 Jul 2012 11:34:35 -0700 | Subject | Re: [tip:perf/core] perf/x86: Fix USER/KERNEL tagging of samples |
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On Fri, Jul 6, 2012 at 11:16 AM, Linus Torvalds <torvalds@linux-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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