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

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Date:	Mon, 16 Mar 2015 15:57:42 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Borislav Petkov <bp@...e.de>
Cc:	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>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC 0/2] x86/fpu: avoid "xstate_fault" in
	xsave_user/xrestore_user

On 03/16, Borislav Petkov wrote:
>
> On Sun, Mar 15, 2015 at 05:49:48PM +0100, Oleg Nesterov wrote:
> >
> > And I agree with Quentin, user_insn/check_insn can be improved to allow
> > clobbers, more flexible "output", etc. But imo they already can make this
> > code look a bit better, and "xstate_fault" must die eventually.
>
> FWIW, I did poke at that but there's still something wrong with my macros, will
> take a look when I get a chance:

Sure, I won't argue if we use the new macros instead. But we already have
check_insn/user_insn, why not use them?

For example,

> +#define XSTATE_XSAVE(st, lmask, hmask, err)				\
> +	asm volatile(ALTERNATIVE_2(XSAVE,				\
> +				   XSAVEOPT, X86_FEATURE_XSAVEOPT,	\
> +				   XSAVES,   X86_FEATURE_XSAVES)	\
> +		     "\n"						\
> +		     ".pushsection .fixup,\"ax\"\n"			\
> +		     "3: movl $-1, %[err]\n"				\
> +		     "jmp " alt_end_marker "b\n"			\
> +		     ".popsection\n"					\
> +		     _ASM_EXTABLE(661b, 3b)				\
> +		     : [err] "=r" (err)					\
> +		     : "D" (st), "a" (lmask), "d" (hmask)		\
> +		     : "memory")
> +

to me check_insn(ALTERNATIVE_2(...)) looks better. Except we need the
clobber. It is not easy to read the code like this, imo it would be better
to avoid copy-and-paste and use the helpers we already have. Just we need
to improve them.


But let me repeat, I leave this to you and others, I do not understand
asm enough.

Oleg.

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