Re: [Qemu-devel] [PATCH 0/3] virtio: don't poll masked vectors with irqfd

[Michael S. Tsirkin]

On Wed, Dec 19, 2012 at 04:53:22PM +0800, Asias He wrote:
> On 12/18/2012 08:39 PM, Michael S. Tsirkin wrote:
> > At the moment when vector is masked virtio will poll it
> > in userspace, even if it is handled by irqfd.
> > This is done in order to update pending bits, but
> > it's not really required until someone reads the pending bits.
> > On the other hand this read results in extra io thread wakeups.
> > 
> > As we only implement the pending bits as a compatibility
> > feature (read - real drivers don't use it), we can defer
> > the irqfd poll until the read actually happens.
> > 
> > This does not seem to affect vhost-net speed
> > in simple benchmarks but could help block: both
> > vhost-blk and dataplane when using irqfd,
> > and I also think this is cleaner than enabling/disabling
> > notifiers all the time.
> > 
> > This will also be the basis for future optimizations.
> > 
> > Michael S. Tsirkin (3):
> >   msi: add API to get notified about pending bit poll
> >   msix: expose access to masked/pending state
> >   virtio-pci: don't poll masked vectors
> > 
> >  hw/pci/msix.c   | 19 +++++++++++++++----
> >  hw/pci/msix.h   |  6 +++++-
> >  hw/pci/pci.h    |  4 ++++
> >  hw/vfio_pci.c   |  2 +-
> >  hw/virtio-pci.c | 53 +++++++++++++++++++++++++++++++++++++++++------------
> >  5 files changed, 66 insertions(+), 18 deletions(-)
> 
> The performance boost is significant here. It is close to the
> result of dropping msix_fire_vector_notifier() hack.

Okay great. I have one other last optimization up my sleeve,
that should hopefully make it as fast as with the hack.

> -- 
> Asias