On Sat, Sep 03, 2011 at 06:19:23PM +0200, Paolo Bonzini wrote: > On 09/02/2011 05:45 PM, Michael S. Tsirkin wrote: > >Well, can you describe an issue in virtio that lfence/sfence help solve > >in terms of a memory model please? > >Pls note that guest uses smp_ variants for barriers. > > /* Make sure buffer is written before we update index. */ > wmb(); > > Without it, a guest could see a partially updated buffer, because > the buffer and index writes are unlocked stores to different > locations.
Sorry, I still don't understand. Yes, they are unlocked stores to different locations. How does it follow that a guest could see a partially updated buffer? Just to make sure - we are talking about x86 here, not ppc, right? > Even if the guest uses barriers, with ioeventfd it will only order > the CPU that is running the guest, not the one that is running the > iothread. In fact I'm surprised that it works at all under x86 with > ioeventfd. > > Paolo I can try to explain if I understand the problem you see. -- MST
