linux-kernel - Re: ASM flags in general

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Message-ID: <F056AAD8-503F-49CB-A9EA-1A3AAB3BBBFA@zytor.com>
Date:	Mon, 27 Jul 2015 16:56:33 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
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

Sure... but now you have to wrap things in stac/clac.  I'm not sure I see the point since the code is already pretty much optimal.

On July 27, 2015 4:49:46 PM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>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.

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.
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