Re: Use of barriers in pvclock ABI

[Avi Kivity]

Glauber Costa wrote:
> On Mon, Aug 11, 2008 at 4:08 AM, Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
>   
> >  Hi,
> >
> >     
> > > However, the pvclock_clocksource_read() implementation is
> > > over-engineered, because it checks for an odd version and uses very
> > > strong rmb() barriers (which generates either an "lfence" or "lock add
> > > $0, (%esp)").
> > >
> > > If we're happy to guarantee as an ABI issue that the record will never
> > > be updated cross-cpu, then we can make the barriers simply barrier() and
> > > just check for (src->version != dst->version).
> > >
> > > Is that OK with you, or do you want to leave open the possibility of
> > > doing cross-cpu time updates?
> > >       
> > Due to the TSC being involved here I don't expect cross-cpu time updates
> > will ever happen.  IMHO it is fine to change that.
> >     
>
> Okay for guest vcpu, but what about physical cpus?
>
>   
> IIRC, the checks are there, and so strict, to account for the
> possiblity of the vcpu to be migrated to another cpu in the middle of
> the
>   


Migration implies an smp barrier (but not a compiler barrier, of 
course).  As to the the version number oddness check, I don't see how it 
can be necessary.


--
error compiling committee.c: too many arguments to function

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