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

[Asias He <asias@redhat.com>]

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.

-- 
Asias