Re: block virtio excessive VIRTIO_PCI_ISR reads

[Michael Tokarev <mjt@tls.msk.ru>]

Anthony Liguori wrote:
> 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.

So the next question is how to use MSI-x.. ;)

Thanks!

/mjt