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SubjectRe: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry
On Sun, Oct 16, 2016 at 05:29:24AM +0300, Michael S. Tsirkin wrote:
> On Sat, Oct 15, 2016 at 03:47:45AM -0400, Paolo Bonzini wrote:
> >
> > > > On Oct 14, 2016, at 11:56 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > > >>>
> > > >>> for (i = 0; i <= 7; i++) {
> > > >>> - pir_val = xchg(&pir[i], 0);
> > > >>> - if (pir_val)
> > > >>> + pir_val = READ_ONCE(pir[i]);
> > > >>
> > > >> Out of curiosity, do you really need this READ_ONCE?
> > > >
> > > > The answer can only be "depends on the compiler's whims". :)
> > > > If you think of READ_ONCE as a C11 relaxed atomic load, then yes.
> > >
> > > Hm.. So the idea is to make the code "race-free” in the sense
> > > that every concurrent memory access is done using READ_ONCE/WRITE_ONCE?
> > >
> > > If that is the case, I think there are many other cases that need to be
> > > changed, for example apic->irr_pending and vcpu->arch.pv.pv_unhalted.
> >
> > There is no documentation for this in the kernel tree unfortunately.
> > But yes, I think we should do that. Using READ_ONCE/WRITE_ONCE around
> > memory barriers is a start.
> >
> > Paolo
>
> I'm beginning to think that if a value is always (maybe except for init
> where we don't much care about the code size anyway) accessed through
> *_ONCE macros, we should just mark it volatile and be done with it. The
> code will look cleaner, and there will be less space for errors
> like forgetting *_ONCE macros.
>
> Would such code (where all accesses are done through
> READ_ONCE/WRITE_ONCE otherwise) be an exception to
> volatile-considered-harmful.txt rules?
>
> Cc Paul and Jonathan (for volatile-considered-harmful.txt).

One concern would be the guy reading the code, to whom it might not
be obvious that the underlying access was volatile, especially if
the reference was a few levels down.

Thanx, Paul

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