linux-kernel - Re: [PATCH RFC 0/2] x86/fpu: avoid "xstate_fault" in xsave_user/xrestore

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Date:	Tue, 17 Mar 2015 10:47:50 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC 0/2] x86/fpu: avoid "xstate_fault" in
 xsave_user/xrestore_user

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.
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