linux-kernel - Re: [PATCH 13/16] x86/entry/64: Create a percpu SYSCALL entry trampoline

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Date:   Mon, 20 Nov 2017 20:34:19 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     X86 ML <x86@...nel.org>, Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 13/16] x86/entry/64: Create a percpu SYSCALL entry
 trampoline

On Mon, Nov 20, 2017 at 09:07:44AM -0800, Andy Lutomirski wrote:
> +	/* Save RDI, since we need a scratch register. */
> +	pushq	%rdi
> +
> +	/*
> +	 * x86 lacks a near absolute jump, and we can't jump to the real
> +	 * entry text with a relative jump, so we use a double trampoline.
> +	 */
> +	movq	$entry_SYSCALL_64_stage2, %rdi
> +	jmp	*%rdi
> +END(entry_SYSCALL_64_trampoline)
> +
> +	.popsection
> +
> +ENTRY(entry_SYSCALL_64_stage2)
> +	/*
> +	 * Rather than polluting the normal SYSCALL path with stack switching
> +	 * nonsense, fix up our register state to match its expectations.
> +	 */
> +	UNWIND_HINT_EMPTY
> +	popq %rdi
> +	jmp entry_SYSCALL_64_after_hwframe
> +END(entry_SYSCALL_64_stage2)

Is there a reason why you couldn't just do the following?

	pushq	$entry_SYSCALL_64_after_hwframe
	ret

Then you wouldn't need the 2nd trampoline and the %rdi clobber.

-- 
Josh