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Re: [PATCH] x86/hweight: Don't clobber %rdi

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On Mon, Aug 8, 2016 at 11:04 AM, Borislav Petkov <bp@alien8.de> wrote:
>
> Hmm, I've been staring at asm for the last hours and my head is spinning
> now, so can you please point me at the exact where this happens. I can't
> find it in x86_perf_event_update().

The corruption could easily have happened long long before.

With a random register clobbered (by a function that is *not* a C
function - it's a replacement for the "popcnt %rdi, %rax"
instruction), you might end up with memory corruption somewhere, and
then an oops much later.

That said, in this case it's likely something like the the
intel_pmu_init() doing

    c->weight = hweight64(c->idxmsk64);

in arch/x86/events/intel/core.c, which then corrupts something related
to the event constraints, and then you get the oops in
x86_perf_event_update() later.

> In any case, it is not a good idea to do the compiler's work. :-\

The compiler has absolutely nothing to do with this. It's all assembly
language and an inline asm.

We *used* to try to have the compiler generate the code. That's what
caused problems.

               Linus

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[PATCH] x86/hweight: Don't clobber %rdi ville.syrjala@linux.intel.com - 2016-08-08 19:40 +0200
  Re: [PATCH] x86/hweight: Don't clobber %rdi Linus Torvalds <torvalds@linux-foundation.org> - 2016-08-08 20:00 +0200
    Re: [PATCH] x86/hweight: Don't clobber %rdi Peter Zijlstra <peterz@infradead.org> - 2016-08-08 20:40 +0200
      Re: [PATCH] x86/hweight: Don't clobber %rdi Jiri Kosina <jikos@kernel.org> - 2016-08-08 21:00 +0200
        Re: [PATCH] x86/hweight: Don't clobber %rdi Borislav Petkov <bp@alien8.de> - 2016-08-08 21:40 +0200
  Re: [PATCH] x86/hweight: Don't clobber %rdi Borislav Petkov <bp@alien8.de> - 2016-08-08 20:10 +0200
    Re: [PATCH] x86/hweight: Don't clobber %rdi Linus Torvalds <torvalds@linux-foundation.org> - 2016-08-08 20:30 +0200
      Re: [PATCH] x86/hweight: Don't clobber %rdi Borislav Petkov <bp@alien8.de> - 2016-08-08 20:40 +0200
        Re: [PATCH] x86/hweight: Don't clobber %rdi Linus Torvalds <torvalds@linux-foundation.org> - 2016-08-08 20:50 +0200
          Re: [PATCH] x86/hweight: Don't clobber %rdi Borislav Petkov <bp@alien8.de> - 2016-08-08 21:00 +0200

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