linux-kernel - Re: [RFC] de-asmify the x86-64 system call slowpath

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Date:	Mon, 27 Jan 2014 16:44:45 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] de-asmify the x86-64 system call slowpath

On Mon, Jan 27, 2014 at 4:22 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 01/27/2014 02:46 PM, Andy Lutomirski wrote:
>>
>> I think that sysret for sigreturn is probably not very interesting.
>> On the other hand, sysret for #PF might be a huge win, despite being
>> even scarier.
>>
>
> SYSRET for #PF or other exceptions is a nonstarter; register state is
> live at that point.

I mean sysret-via-trampoline for #PF.

It's scary, it probably has issues with ptrace and interrupts that hit
while the trampoline is still running, and it could break anything
that writes past the red zone, but I think it could work.

No, I don't particularly want to implement (and debug) such a beast.

(I will continue cursing Intel -- why can't we have a fast way to
return to 64-bit userspace with complete control of all non-segment
registers?  sysret is *almost* the right thing.)

--Andy

>
>         -hpa
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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