linux-kernel - Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti

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Date:   Wed, 10 Jan 2018 09:22:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Willy Tarreau <w@....eu>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set

On Tue, Jan 09, 2018 at 01:56:20PM +0100, Willy Tarreau wrote:
>   - use pti_disable instead of task flag
> ---
>  arch/x86/entry/calling.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 2c0d3b5..5361a10 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -229,6 +229,11 @@
>  
>  .macro SWITCH_TO_USER_CR3_NOSTACK scratch_reg:req scratch_reg2:req
>  	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
> +
> +	/* The "pti_disable" mm attribute is mirrored into this per-cpu var */
> +	cmpb	$0, PER_CPU_VAR(pti_disable)
> +	jne	.Lend_\@
> +
>  	mov	%cr3, \scratch_reg

So could you switch back to a task flag for this? That word is already
cache-hot on the exit path while your new variable is not.