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 08:15:10 +0100
From:   Ingo Molnar <mingo@...nel.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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.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


* Willy Tarreau <w@....eu> wrote:

> When a syscall returns to userspace with pti_disable set, it means the
> current mm is configured to disable page table isolation (PTI). In this
> case, returns from kernel to user will not switch the CR3, leaving it
> to the kernel one which already maps both user and kernel pages. This
> avoids a TLB flush, and saves another one on next entry.
> 
> Thanks to these changes, haproxy running under KVM went back from
> 12700 conn/s (without PCID) or 19700 (with PCID) to 23100 once loaded
> after calling prctl(), indicating that PTI has no measurable impact on
> this workload.
> 
> Signed-off-by: Willy Tarreau <w@....eu>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Brian Gerst <brgerst@...il.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Kees Cook <keescook@...omium.org>
> 
> v2:
>   - 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_\@

Could you please do this small change for future iterations:

s/per-cpu
 /per-CPU

... to make the spelling more consistent with the rest of the code base?

Thanks,

	Ingo