linux-kernel - Re: [RFC/HACK] x86: Fast return to kernel

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Date:	Fri, 2 May 2014 14:04:30 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	"H. Peter Anvin" <h.peter.anvin@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC/HACK] x86: Fast return to kernel

On Fri, May 2, 2014 at 2:01 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, May 2, 2014 at 1:30 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> So what about manipulating the stack so that the popf does not enable
>> interrupts and do an explicit sti to get the benefit of the
>> one-instruction shadow ?
>
> That's what I already suggested in the original "I don't think popf
> works" email.
>
> It does get more complex since you now have to test things (there are
> very much cases where we get page faults and other exceptions with
> interrupts disabled), but it shouldn't be much worse.
>
> Btw, Andy, why did you do "popq %rsp"? That just looks crazy. If the
> stack isn't contiguous, the subsequent "popf" couldn't have worked
> anyway. And I bet it screws with the stack engine. So you should just
> have done something like "addq $16,%rsp" or whatever the constant ends
> up being.

Because otherwise I'd have to keep track of whether it's a zeroentry
or an errorentry.  I can't stuff the offset in a register without even
more stack hackery, since there are no available registers there.  I
could split the whole thing into two code paths, I guess.

--Andy
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