Re: [PATCH 4/6] KVM: Add kvm_get_irq_routing_entry() func

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 18, 2010 at 07:59:10PM +0800, Sheng Yang wrote:
> On Thu, Nov 18, 2010 at 5:41 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Thu, Nov 18, 2010 at 11:30:47AM +0200, Avi Kivity wrote:
> >> >>
> >> >>  *entry may be stale after rcu_read_unlock().  Is this a problem?
> >> >
> >> >I suppose not. All MSI-X MMIO accessing would be executed without delay, so no re-
> >> >order issue would happen. If the guest is reading and writing the field at the same
> >> >time(from two cpus), it should got some kinds of sync method for itself - or it
> >> >may not care what's the reading result(like the one after msix_mask_irq()).
> >>
> >> I guess so.  Michael/Alex?
> >
> > This is kvm_get_irq_routing_entry which is used for table reads,
> > correct?  Actually, the pci read *is* the sync method that guests use,
> > they rely on reads to flush out all previous writes.
> 
> Michael, I think the *sync* you are talking about is not the one I
> meant. I was talking about two cpus case, one is reading and the other
> is writing, the order can't be determined if guest doesn't use lock or
> some other synchronize methods; and you're talking about to flush out
> all previous writes of the only one CPU...

Yes, but you don't seem to flush out writes on a read, either.

> -- 
> regards,
> Yang, Sheng