Re: [Qemu-devel] [PATCH v5 23/45] migrate_start_postcopy: Command to trigger transition to postcopy

[Dr. David Alan Gilbert]

* Paolo Bonzini (address@hidden) wrote:
> 
> 
> On 30/03/2015 19:04, Dr. David Alan Gilbert wrote:
> >>> > > That one's a trickier question.  Compilers are absolutely capable
> >>> > > of optimizing that far, *but* the C rules about when it's allowed
> >>> > > to assume in-memory values remain unchanged are pretty
> >>> > > conservative.  I think any function call in the loop will require
> >>> > > it to reload the value, for example.  That said, a (compiler only)
> >>> > > memory barrier might be appropriate to ensure that reload.
> >> > 
> >> > That's exactly what atomic_read provides.
> > So does that say I need the atomic_read but not the atomic_write -
> > which seems a bit weird, but I think only due to the naming.
> 
> No, you need both even though it's even more far-fetched that the
> compiler will do something bad with the set.

OK, done - it's back to where it was with atomic_set/atomic_read.

Dave

> 
> Paolo
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK