Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
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- Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
- From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 15 Jun 2009 12:04:42 -0700 (PDT)
- Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>, mingo@xxxxxxxxxx, hpa@xxxxxxxxx, paulus@xxxxxxxxx, acme@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, a.p.zijlstra@xxxxxxxxx, penberg@xxxxxxxxxxxxxx, vegard.nossum@xxxxxxxxx, efault@xxxxxx, jeremy@xxxxxxxx, npiggin@xxxxxxx, tglx@xxxxxxxxxxxxx, linux-tip-commits@xxxxxxxxxxxxxxx
- In-reply-to: <20090615183649.GA16999@xxxxxxx>
- References: <tip-74193ef0ecab92535c8517f082f1f50504526c9b@xxxxxxxxxxxxxx> <alpine.LFD.2.01.0906151007560.3305@xxxxxxxxxxxxxxxxxxxxx> <20090615171845.GA7664@xxxxxxx> <alpine.LFD.2.01.0906151029160.3305@xxxxxxxxxxxxxxxxxxxxx> <20090615180527.GB4201@Krystal> <alpine.LFD.2.01.0906151125320.6276@xxxxxxxxxxxxxxxxxxxxx> <20090615183649.GA16999@xxxxxxx>
- User-agent: Alpine 2.01 (LFD 1184 2008-12-16)
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
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