Re: [Xen-devel] [PATCH] x86: fix wait code asm() constraints

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.08.12 at 14:08, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
> On 03/08/2012 12:36, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> 
>>> I'm confused. The registers have the same values at the start and the end of
>>> the asm statement. How can it possibly matter, even in theory, whether they
>>> temporarily change in the middle? Is this fairly strong assumption written
>>> down in the gcc documentation anywhere?
>> 
>> It's in the specification of the & modifier:
>> 
>> "Å&Â Means (in a particular alternative) that this operand is an
>>  earlyclobber operand, which is modified before the instruction
>>  is finished using the input operands. Therefore, this operand
>>  may not lie in a register that is used as an input operand or as
>>  part of any memory address."
>> 
>> Of course, here we're not having any other operands, which
>> is why at least at present getting this wrong does no harm.
> 
> Yep, okay, that makes sense. Especially the use of an input operand to form
> a memory address, although of course that cannot happen in our specific case
> here. I have acked your patch, although I'd like an update to the patch
> comment.

How about this as an added initial paragraph?

This fixes theoretical issues with those constraints - operands that
get clobbered before consuming all input operands must be marked so
according the the gcc documentation. Beyond that, the change is merely
code improvement, not a bug fix.

Jan

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