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Date:   Sat, 2 Dec 2017 09:18:40 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 4/6] Unsuck "x86/entry/64: Create a percpu SYSCALL entry
 trampoline"

On Thu, Nov 30, 2017 at 10:29:44PM -0800, Andy Lutomirski wrote:
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index caf74a1bb3de..28f4e7553c26 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -180,14 +180,24 @@ ENTRY(entry_SYSCALL_64_trampoline)
>  
>  	/*
>  	 * x86 lacks a near absolute jump, and we can't jump to the real
> -	 * entry text with a relative jump, so we fake it using retq.
> +	 * entry text with a relative jump.  We could push the target
> +	 * address and then use retq, but this destroys the pipeline on
> +	 * many CPUs (wasting over 20 cycles on Sandy Bridge).  Instead,
> +	 * spill RDI and restore it in a second-stage trampoline.
>  	 */
> -	pushq	$entry_SYSCALL_64_after_hwframe
> -	retq
> +	pushq	%rdi
> +	movq	$entry_SYSCALL_64_stage2, %rdi
> +	jmp	*%rdi
>  END(entry_SYSCALL_64_trampoline)
>  
>  	.popsection
>  
> +ENTRY(entry_SYSCALL_64_stage2)
> +	UNWIND_HINT_EMPTY
> +	popq	%rdi
> +	jmp	entry_SYSCALL_64_after_hwframe
> +END(entry_SYSCALL_64_stage2)
> +
>  ENTRY(entry_SYSCALL_64)
>  	UNWIND_HINT_EMPTY
>  	/*

Another crazy idea:

	call	1f
1:	movq	$entry_SYSCALL_64_after_hwframe, (%rsp)
	ret

Does that fix the regression?

-- 
Josh

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