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

Linus Torvalds
2009-06-15T19:05:56Z

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.

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