Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

Mathieu Desnoyers
2009-06-15T19:39:56Z

* Linus Torvalds (torvalds@linux-foundation.org) wrote:
>
>
> On Mon, 15 Jun 2009, Ingo Molnar wrote:
> >
> > The gist of it is the replacement of iret with this open-coded
> > sequence:
> >
> > +#define NATIVE_INTERRUPT_RETURN_NMI_SAFE pushq %rax; \
> > + movq %rsp, %rax; \
> > + movq 24+8(%rax), %rsp; \
> > + pushq 0+8(%rax); \
> > + pushq 16+8(%rax); \
> > + movq (%rax), %rax; \
> > + popfq; \
> > + ret
>
> That's an odd way of writing it.
>

There were a few reasons (maybe not all good) for writing it like this :

- Saving I$ (as it is placed close to hot entry.S code paths)
- Staying localized with the top of stack, saving D$ accesses.

But maybe benchmarks will prove my approach overkill, dunno. Also we
have to be aware that the CPU might behave more slowly in the presence
of unbalanced int/iret, call/ret. I think we should benchmark your
approach to make sure jmp will not produce such slowdown. But it might
well be faster, and it's definitely clearer.

Thanks,

Mathieu

> Don't we have a per-cpu segment here? I'd much rather just see it do
> something like this (_before_ restoring the regular registers)
>
> movq EIP(%esp),%rax
> movq ESP(%esp),%rdx
> movq %rax,gs:saved_esp
> movq %rdx,gs:saved_eip
>
> # restore regular regs
> RESTORE_ALL
>
> # skip eip/esp to get at eflags
> addl $16,%esp
> popfq
>
> # restore rsp/rip
> movq gs:saved_esp,%rsp
> jmpq *(gs:saved_eip)
>
> but I haven't thought deeply about it. Maybe there's something wrong with
> the above.
>
> > If it's faster, this becomes a legit (albeit complex)
> > micro-optimization in a _very_ hot codepath.
>
> I don't think it's all that hot. It's not like it's the return to user
> mode.
>
> Linus

--
Mathieu Desnoyers
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