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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 14 Jun 2017, Jan Beulich wrote:
> >>> 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.

The relation is that our "compiler expectations" shape what compilers we
support.

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