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Date: Tue, 17 Mar 2015 11:07:36 +0100
From: Quentin Casasnovas <quentin.casasnovas@oracle.com>
To: Borislav Petkov <bp@suse.de>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>,
        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>,
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        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: <20150317100736.GB19131@chrystal.uk.oracle.com>
References: <54F74F59.5070107@intel.com>
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 <20150316223743.GA14575@chrystal.uk.oracle.com>
 <20150317094750.GD18917@pd.tnic>
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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