[Qemu-devel] How many msi-x vectors should be allocated for the virtio-serial device?

[Gal Hammer]

Hi,

How many msi-x vectors should be allocated for the virtio-serial device?

I'm asking this as it seems that a proposed patch 
(http://lists.gnu.org/archive/html/qemu-devel/2011-12/msg02094.html) was 
not accepted and I re-encountered this issue while trying to upgrade the 
virtio-serial's Windows driver to use msi-x vectors.

The virtio-serial's Linux module tries to use one vector per virtqueue 
(every serial port have two virtqueues) with a fall back to using only 
two vectors 
(http://lxr.linux.no/linux+v3.7.2/drivers/virtio/virtio_pci.c#L539). The 
problem is that qemu's virtio-pci device allocate less vectors than the 
modules expects. So, for example, if a serial device have 16 ports, 17 
vectors are allocated. The module tries to use 34 vectors, fails and 
choose to use only 2, leaving 15 unused vectors.

Is it possible to increase the vectors number from 
"proxy->serial.max_virtserial_ports + 1" to 
"(proxy->serial.max_virtserial_ports + 1) * 2"?

Thanks,

    Gal.