linux-kernel - Re: ASM flags in general

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Date:	Mon, 27 Jul 2015 16:36:26 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Uros Bizjak <uros_bizjak1@....net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Uros Bizjak <ubizjak@...il.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: ASM flags in general

On Mon, Jul 27, 2015 at 4:22 PM, H. Peter Anvin <hpa@...or.com> wrote:
> For that to work, gcc would have to know about the extable.

It could, I think:

asm goto (
    "1: mov ...\n\t"
    _ASM_EXTABLE(1b, %l2)  /* or whatever index it is */
    : ... : ... : ... : efault);

return 0;

efault:
     return -EFAULT;

I think that wrmsr_safe could get this treatment with current GCC.
put_user plausibly could, too, if we were willing to mark it volatile
and accept that we're lying a little bit about the lack of an output
constraint.  get_user would need GCC to understand output constraints
for real.

--Andy
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