linux-kernel - Re: [PATCH 5/6] x86/mm/kaiser: Optimize RESTORE

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Date:   Wed, 29 Nov 2017 21:02:12 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>, daniel.gruss@...k.tugraz.at,
        hughd@...gle.com, keescook@...gle.com, linux-mm@...ck.org,
        michael.schwarz@...k.tugraz.at, moritz.lipp@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at
Subject: Re: [PATCH 5/6] x86/mm/kaiser: Optimize RESTORE_CR3

On Wed, Nov 29, 2017 at 11:33:06AM +0100, Peter Zijlstra wrote:
> Currently RESTORE_CR3 does an unconditional flush
> (SAVE_AND_SWITCH_TO_KERNEL_CR3 does not set bit 63 on \save_reg).
> 
> When restoring to a user ASID, check the user_asid_flush_mask to see
> if we can avoid the flush.
> 
> For kernel ASIDs we can unconditionaly avoid the flush, since we do
> explicit flushes for them.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/entry/calling.h  |   29 +++++++++++++++++++++++++++--
>  arch/x86/entry/entry_64.S |    4 ++--
>  2 files changed, 29 insertions(+), 4 deletions(-)
> 
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -263,8 +263,33 @@ For 32-bit we have the following convent
>  .Ldone_\@:
>  .endm
>  
> -.macro RESTORE_CR3 save_reg:req
> +.macro RESTORE_CR3 scratch_reg:req save_reg:req
>  	STATIC_JUMP_IF_FALSE .Lend_\@, kaiser_enabled_key, def=1
> +
> +	/* ASID bit 11 is for user */
> +	bt	$11, \save_reg

<---- newline here.

> +	/*
> +	 * KERNEL pages can always resume with NOFLUSH as we do
> +	 * explicit flushes.
> +	 */
> +	jnc	.Lnoflush_\@
> +
> +	/*
> +	 * Check if there's a pending flush for the user ASID we're
> +	 * about to set.
> +	 */
> +	movq	\save_reg, \scratch_reg
> +	andq	$(0x7FF), \scratch_reg
> +	bt	\scratch_reg, PER_CPU_VAR(user_asid_flush_mask)
> +	jnc	.Lnoflush_\@
> +
> +	btr	\scratch_reg, PER_CPU_VAR(user_asid_flush_mask)
> +	jmp	.Ldo_\@

Can you save yourself one of the BT-insns?

	...
	andq	$(0x7FF), \scratch_reg
	btr     \scratch_reg, PER_CPU_VAR(user_asid_flush_mask)
	jnc	.Lnoflush_\@
	jmp     .Ldo_\@
	...

or am I missing a case?

-- 
Regards/Gruss,
    Boris.

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