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 12:36:58 +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 12:20:15PM +0100, Borislav Petkov wrote:
> On Tue, Mar 17, 2015 at 11:00:46AM +0100, Quentin Casasnovas wrote:
> > Fair point, but AFAIUI we can't do check_insn(XSAVES) alone as of today,
> > and the "..." in your "check_isns(XSAVEOPT, ...)" code above would still
> > need to contain the outputs operands.
> 
> I think we can do this (see diff the end of this mail).
>

Right, FWIW I think your approach is valid, but not very generic.  Re-using
the check_insn() and making it more generic so we can widen its use felt
like a better approach to me.

AIUI, you didn't like my earlier draft because it wasn't very readable, but
I think this was just due to the (bad) example I took and by reworking it a
bit more, we could end up with the code you previously envisionned:

  if (static_cpu_has_safe(X86_FEATURE_XSAVEOPT))
          return check_insn(XSAVEOPT, xsave_buf, ...);
  else if (static_cpu_has_safe(X86_FEATURE_XSAVES)
          return check_insn(XSAVES, xsave_buf, ...);
  else
	  return check_insn(XSAVE, xsave_buf, ...)

Or maybe you were saying the actual macros weren't readable?

> [...]
> 
> But including the fault exception table in the macro is already an
> improvement IMO.

Agreed, it already looks much nicer with your diff.

Quentin
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