On Tue, Nov 26, 2013 at 06:06:26PM +0200, Avi Kivity wrote: > On 11/26/2013 05:58 PM, Paolo Bonzini wrote: > >Il 26/11/2013 16:35, Avi Kivity ha scritto: > >>>>>If we want to ensure, we need to use a different mechanism for > >>>>>synchronization than the global RCU. QRCU would work; readers are not > >>>>>wait-free but only if there is a concurrent synchronize_qrcu, which > >>>>>should be rare. > >>>>An alternative path is to convince ourselves that the hardware does not > >>>>provide the guarantees that the current code provides, and so we can > >>>>relax them. > >>>No, I think it's a reasonable guarantee to provide. > >>Why? > >Because IIUC the semantics may depend not just on the interrupt > >controller, but also on the specific PCI device. It seems safer to > >assume that at least one device/driver pair wants this to work. > > It's indeed safe, but I think there's a nice win to be had if we > drop the assumption. I'm not arguing with that, but a minor commoent below: > >(BTW, PCI memory writes are posted, but configuration writes are not). > > MSIs are configured via PCI memory writes. > > By itself, that doesn't buy us anything, since the guest could flush > the write via a read. But I think the fact that the interrupt > messages themselves are posted proves that it is safe. FYI, PCI read flushes the interrupt itself in, too. > The fact > that Linux does interrupt migration from within the interrupt > handler also shows that someone else believes that it is the only > safe place to do it. > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
