Re: [PATCH 03/10] x86/entry/32: Filter NT and speed up AC filtering in SYSENTER

[Borislav Petkov]

On Sun, Feb 28, 2016 at 09:28:48PM -0800, Andy Lutomirski wrote:
> This makes the 32-bit code work just like the 64-bit code.  It should
> speed up syscalls on 32-bit kernels on Skylake by something like 20
> cycles (by analogy to the 64-bit compat case).
> 
> It also cleans up NT just like we do for the 64-bit case.
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  arch/x86/entry/entry_32.S | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
> index ab710eee4308..263ebde6333f 100644
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -294,7 +294,6 @@ sysenter_past_esp:
>  	pushl	$__USER_DS		/* pt_regs->ss */
>  	pushl	%ebp			/* pt_regs->sp (stashed in bp) */
>  	pushfl				/* pt_regs->flags (except IF = 0) */
> -	ASM_CLAC			/* Clear AC after saving FLAGS */
>  	orl	$X86_EFLAGS_IF, (%esp)	/* Fix IF */
>  	pushl	$__USER_CS		/* pt_regs->cs */
>  	pushl	$0			/* pt_regs->ip = 0 (placeholder) */
> @@ -302,6 +301,23 @@ sysenter_past_esp:
>  	SAVE_ALL pt_regs_ax=$-ENOSYS	/* save rest */
>  
>  	/*
> +	 * Sysenter doesn't filter flags, so we need to clear NT and AC
> +	 * ourselves.  To save a few cycles, we can check whether
> +	 * either was set instead of doing an unconditional popfq.

Let's write insn names capitalized: SYSENTER, POPF, etc... I know,
entry_SYSENTER_compat() doesn't do it either but you could fix that up
in your 2/10 patch, since you're touching that place anyway...

-- 
Regards/Gruss,
    Boris.

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