On Thu, Nov 12, 2009 at 01:18:11AM +0800, Scott Tsai wrote: > 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
Hmm, on x86 that's more expensive than it needs to be... We can also ifdef platforms that we care about ... > 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
