Re: block virtio excessive VIRTIO_PCI_ISR reads

[Anthony Liguori <anthony@codemonkey.ws>]

Saul Tamari wrote:
> Hi,
>
> There seems to be a bug in the VirtIO guest device driver (tested with
> 2.6.31.5 kernel).
> The bug causes frequent & excessive VIRTIO_PCI_ISR reads (causing PIO
> vm-exits) for a device that is not being used.
>
> The setup I’m using includes two VirtIO block devices – vda & vdb.
> While running heavy IO on vdb I notice a lot of VIRTIO_PCI_ISR port
> reads for the PIO ports of vda too.
> For each vdb IO operation passed to the hypervisor by the virtio driver I see:
> 1. One VIRTIO_PCI_QUEUE_NOTIFY write on vdb
> 2. One VIRTIO_PCI_ISR read on vdb
> 3. One VIRTIO_PCI_ISR read on vda (while not explicitly using vda)
>   

That's because it's using a shared interrupt and each device needs to 
check whether it's got pending work to do.

You can eliminate this by using MSI-x.

Regards,

Anthony Liguori