linux-kernel - Re: [PATCH v2] x86: Return to kernel without IRET

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Message-ID: <CA+55aFwmhOGRv0Z7Sqgk08pTOYDJg4C9GB_XNHRAtiCLFrHG1g@mail.gmail.com>
Date:	Fri, 2 May 2014 21:32:53 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v2] x86: Return to kernel without IRET

On Fri, May 2, 2014 at 4:53 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On my box, this saves about 100ns on each interrupt and trap that
> happens while running in kernel space.  This speeds up my kernel_pf
> microbenchmark by about 17%.

Btw, would you mind _trying_ to do a similar trick for the "return to
user space" case?

At least as a proof-of-concept, having a code sequence in user mode
trampoline that does

   popq %rsi
   popq %r11
   retq $128

and building up a stack in user space at '%rsp-128' that has the
values or rsi/r11/rip should allow us to use 'sysret'. Hmm?

            Linus
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