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

Michael S. Tsirkin
2012-12-19T11:26:40Z

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