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 11:07:36 +0100
From:	Quentin Casasnovas <quentin.casasnovas@...cle.com>
To:	Borislav Petkov <bp@...e.de>
Cc:	Quentin Casasnovas <quentin.casasnovas@...cle.com>,
	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 Tue, Mar 17, 2015 at 10:47:50AM +0100, Borislav Petkov wrote:
> 
> 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.
> 

What was the argument against adding a check_alternative_input(...) so the
ex_table entry are managed inside the macro directly?  It leaves less room
for errors and would still be reable IMO:

err = check_alternative_input_2(XSAVE,
				XSAVESOPT, X86_FEATURE_XSAVEOPT
      				XSAVES, X86_FEATURE_XSAVES,
				<inputs>, <outputs>, <clobbers>);
if (err)
   do_something();

That hypothetical check_alternative_input_2() would call a rework of
check_insn() supporting an arbitrary numbers of inputs, outputs and
clobbers as drafted in my previous e-mail.

Quentin
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