linux-kernel - Re: [tip:x86/asm] x86/asm/entry/32: Document our abuse of x86_hw_tss: :ss1 and x86_hw

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Message-ID: <CALCETrW9FRCiQdaktv+NYZE==umYu91eso3d7VV9B5VgQMD1Sw@mail.gmail.com>
Date:	Mon, 16 Mar 2015 08:36:47 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/asm] x86/asm/entry/32: Document our abuse of x86_hw_tss:
 :ss1 and x86_hw_tss::sp1

On Mon, Mar 16, 2015 at 5:09 AM, tip-bot for Andy Lutomirski
<tipbot@...or.com> wrote:
> Commit-ID:  f7113ffa3bcd630066119723f539db75a5721c88
> Gitweb:     http://git.kernel.org/tip/f7113ffa3bcd630066119723f539db75a5721c88
> Author:     Andy Lutomirski <luto@...capital.net>
> AuthorDate: Tue, 10 Mar 2015 11:06:00 -0700
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Mon, 16 Mar 2015 11:05:36 +0100
>
> x86/asm/entry/32: Document our abuse of x86_hw_tss::ss1 and x86_hw_tss::sp1
>
> This has confused me for a while.  Now that I figured it out, document it.
>
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Denys Vlasenko <dvlasenk@...hat.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Link: http://lkml.kernel.org/r/b7efc1b7364039824776f68e9ddee9ec1500e894.1426009661.git.luto@amacapital.net
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> ---
>  arch/x86/include/asm/processor.h | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index aed6d4f..c3a037b 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -209,9 +209,24 @@ struct x86_hw_tss {
>         unsigned short          back_link, __blh;
>         unsigned long           sp0;
>         unsigned short          ss0, __ss0h;
> -       unsigned long           sp1;
> -       /* ss1 caches MSR_IA32_SYSENTER_CS: */
> -       unsigned short          ss1, __ss1h;
> +
> +       /*
> +        * We don't use ring 1, so sp1 and ss1 are convenient scratch
> +        * spaces in the same cacheline as sp0.  We use them to cache
> +        * some MSR values to avoid unnecessary wrmsr instructions.
> +        *
> +        * We use SYSENTER_ESP to find sp0 and for the NMI emergency
> +        * stack, but we need to context switch it because we do
> +        * horrible things to the kernel stack in vm86 mode.

I should have sent out my v2 sooner.  Denys correctly noted that the
sp1 cache is completely useless.  I'll send an incremental patch
today.

--Andy

> +        *
> +        * We use SYSENTER_CS to disable sysenter in vm86 mode to avoid
> +        * corrupting the stack if we went through the sysenter path
> +        * from vm86 mode.
> +        */
> +       unsigned long           sp1;    /* MSR_IA32_SYSENTER_ESP */
> +       unsigned short          ss1;    /* MSR_IA32_SYSENTER_CS */
> +
> +       unsigned short          __ss1h;
>         unsigned long           sp2;
>         unsigned short          ss2, __ss2h;
>         unsigned long           __cr3;



-- 
Andy Lutomirski
AMA Capital Management, LLC
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