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

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Message-ID: <alpine.DEB.2.02.1405022222570.6261@ionos.tec.linutronix.de>
Date:	Fri, 2 May 2014 22:30:44 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Andy Lutomirski <luto@...capital.net>,
	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, 2 May 2014, Linus Torvalds wrote:

> On Fri, May 2, 2014 at 12:31 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Also, are you *really* sure that "popf" has the same one-instruction
> > interrupt shadow that "sti" has? Because I'm not at all sure that is
> > true, and it's not documented as far as I can tell. In contrast, the
> > one-instruction shadow after "sti" very much _is_ documented.
> 
> Yeah, I'm pretty sure about this. The only instructions with an
> interrupt shadow are "sti", "mov ss" and "pop ss".
> 
> There may be specific microarchitectures that do it for a "popf" that
> enables interrupts too, but that is not documented _anywhere_ I could
> find.

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 ?

Thanks,

	tglx
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