From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752361AbdKUCeX (ORCPT <rfc822;w@1wt.eu>);
        Mon, 20 Nov 2017 21:34:23 -0500
Received: from mx1.redhat.com ([209.132.183.28]:57184 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1751389AbdKUCeV (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Mon, 20 Nov 2017 21:34:21 -0500
Date: Mon, 20 Nov 2017 20:34:19 -0600
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: X86 ML <x86@kernel.org>, Borislav Petkov <bpetkov@suse.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>, Dave Hansen <dave.hansen@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 13/16] x86/entry/64: Create a percpu SYSCALL entry
 trampoline
Message-ID: <20171121023419.vs6mpg3ip5xklmvl@treble>
References: <cover.1511195781.git.luto@kernel.org>
 <092b216eb666173f435ad6f8848bfc0069ff7714.1511195781.git.luto@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <092b216eb666173f435ad6f8848bfc0069ff7714.1511195781.git.luto@kernel.org>
User-Agent: Mutt/1.6.0.1 (2016-04-01)
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Tue, 21 Nov 2017 02:34:21 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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