linux-kernel - Re: ASM flags in general

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Date:	Mon, 27 Jul 2015 16:49:46 -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 4:46 PM, H. Peter Anvin <hpa@...or.com> wrote:
> Sure, but that is different than getting rid of the _ex forms.
>

If we did that and got rid of the _ex forms, though, then the code
that matters (the no-fault case) would just be a bunch of movs, right?
 That's basically the same as the current _ex code.

--Andy

> On July 27, 2015 4:36:26 PM PDT, Andy Lutomirski <luto@...capital.net>
> wrote:
>>
>> 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
>
>
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.



-- 
Andy Lutomirski
AMA Capital Management, LLC
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