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

[Mathieu Desnoyers]

* Linus Torvalds (torvalds@xxxxxxxxxxxxxxxxxxxx) 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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