On Wed, Nov 11, 2009 at 10:08 PM, Michael S. Tsirkin <m...@redhat.com> wrote: > On Wed, Nov 11, 2009 at 01:45:35PM +0000, Paul Brook wrote: >> If you don't need real barriers, then why does the kvm code have them? > > We need real barriers but AFAIK kvm does not have them :( > IOW: virtio is currently broken with kvm, and my patch did > not fix this. The comment that I added says as much.
How about just using GCC's __sync__synchronize atomic builtin (if detected as available by configure)? It's a full memory barrier instead of just a write barrier, for x86, it generates the same code as the current Linux mb() implementation: "mfence" on x86_64 "lock orl $0x0,(%esp)" on x86 unless -march is specified to a processor with "mfence". PPC could continue to use "eieio" while other architectures could just default to __sync_synchronize I do have a newbie question, when exactly would vrtio have to handle concurrent access from multiple threads? My current reading of the code suggests: 1. when CONFIG_IOTHREAD is true 2. when CONFIG_KVM is true and the guest machine has multiple CPUs
