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

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Date:	Mon, 15 Jun 2009 13:14:45 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	mingo@...hat.com, hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	penberg@...helsinki.fi, vegard.nossum@...il.com, efault@....de,
	npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support
 to use NMI-safe methods

On 06/15/09 12:04, Linus Torvalds wrote:
> 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.
>    

We have to restore the usermode %gs somewhere...

     J
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