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 S934520AbbCQJu0 (ORCPT <rfc822;w@1wt.eu>);
	Tue, 17 Mar 2015 05:50:26 -0400
Received: from cantor2.suse.de ([195.135.220.15]:36929 "EHLO mx2.suse.de"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S932486AbbCQJtZ (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Tue, 17 Mar 2015 05:49:25 -0400
Date: Tue, 17 Mar 2015 10:47:50 +0100
From: Borislav Petkov <bp@suse.de>
To: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Oleg Nesterov <oleg@redhat.com>, Dave Hansen <dave.hansen@intel.com>,
        Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Pekka Riikonen <priikone@iki.fi>, Rik van Riel <riel@redhat.com>,
        Suresh Siddha <sbsiddha@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH RFC 0/2] x86/fpu: avoid "xstate_fault" in
 xsave_user/xrestore_user
Message-ID: <20150317094750.GD18917@pd.tnic>
References: <54F74F59.5070107@intel.com>
 <20150315164948.GA28149@redhat.com>
 <20150316223743.GA14575@chrystal.uk.oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20150316223743.GA14575@chrystal.uk.oracle.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, Mar 16, 2015 at 11:37:44PM +0100, Quentin Casasnovas wrote:

...

>   __user_insn("btl [var2], %0		\n\t",
>   	      , /* no outputs, no need for dummy arg */
> 	      SINGLE_ARG("r" (var1), [var2] "r" (var2)), /* two inputs */
> 	      "cc");

So this becomes pretty unreadable IMO. And we shouldn't go nuts with
optimizing this and sacrifice readability a lot.

TBH, I'd much prefer:

	if (static_cpu_has_safe(X86_FEATURE_XSAVEOPT)) {
		check_insn(XSAVEOPT, ...);
		return;
	}

	if (static_cpu_has_safe(X86_FEATURE_XSAVES)) {
		check_insn(XSAVES);
		return;
	}

	check_insn(XSAVE, ...)

which is pretty clear.

We can even go a step further and add a static_cpu_has_safe thing which
checks two features instead of one. The penalty we'd get is a single
inconditional JMP which in the face of XSAVE* is nothing.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--