Re: [Xen-devel] [RFC PATCH] docs: add README.atomic

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Stefano Stabellini <sstabellini@xxxxxxxxxx> 06/14/17 8:45 PM >>>
>On Wed, 14 Jun 2017, Jan Beulich wrote:
>> > +What ACCESS_ONCE does *not* guarantee though is this access is done in a
>> > +single instruction, so complex or non-native or unaligned data types are
>> > +not guaranteed to be atomic. If for instance counter would be a 64-bit 
>> > value
>> > +on a 32-bit system, the compiler would probably generate two load 
>> > instructions,
>> > +which could end up in reading a wrong value if some other CPU changes the 
>> > other
>> > +half of the variable in between those two reads.
>> > +However accessing _aligned and native_ data types is guaranteed to be 
>> > atomic
>> > +in the architectures supported by Xen, so ACCESS_ONCE is safe to use when
>> > +these conditions are met.
>> 
>> As mentioned before, such a guarantee does not exist. Please only
>> state what is really the case, i.e. we _expect_ compilers to behave
>> this way.
>
>Regarding compilers support: do we state clearly in any docs or website
>what are the compilers we actually support? I think this would be the
>right opportunity to do it.

At the very least we state somewhere what gcc versions we support. However,
I can't see the relation of such a statement to the discussion here.

Jan



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