Virtualization - Oct 2019 - question: asynchronous notification from vhost

Hi,

I'm developing a virtualised audio / DSP virtio and vhost driver pair 
and I'm currently somewhat stuck trying to figure out how to 
asynchronously notify the guest from the vhost driver. I'm using the 
vhost_add_used_and_signal() function to return data back to the guest 
in the guest context, when the guest initiated an operation, that's 
working well. But how do I "kick" the guest from an asynchronous, e.g.
from an interrupt thread context?

Thanks
Guennadi

On Tue, Oct 15, 2019 at 11:23:13AM +0200, Guennadi Liakhovetski
wrote:
> Hi,
> 
> I'm developing a virtualised audio / DSP virtio and vhost driver pair 
> and I'm currently somewhat stuck trying to figure out how to 
> asynchronously notify the guest from the vhost driver. I'm using the 
> vhost_add_used_and_signal() function to return data back to the guest 
> in the guest context, when the guest initiated an operation, that's 
> working well. But how do I "kick" the guest from an asynchronous,
e.g.
> from an interrupt thread context?
I think I've solved that by using a vhost work queue. That brings me 
one step further, possibly, to the last larger problem to solve: the 
actual data transmission. My preference would be to use zero-copy 
for that, but so far I only see one example of zero-copy in 
drivers/vhost - in net.c and only in TX direction. As far as I 
understand that isn't a principal limitation of the vhost / virtio 
API, rather it's related to certain networking specifics. Am I 
right? Is it supposed to work in both firections, or are there 
problems?

Thanks
Guennadi