RE: [PATCH v1 0/2] Virtio: fix some vq allocation issues

[Wang, Wei W]

On Sunday, December 30, 2018 2:06 PM, Halil Pasic wrote:
> 
> I guess you are the first one trying to read virtio config from within interrupt
> context. AFAICT this never worked.

I'm not sure about "never worked". It seems to work well with virtio-pci.
But looking forward to hearing a solid reason why reading config inside
the handler is forbidden (if that's true).

> About what happens. The apidoc of ccw_device_start() says it needs to be
> called with the ccw device lock held, so ccw_io_helper() tries to take it (since
> forever I guess). OTOH do_cio_interrupt() takes the subchannel lock and
> io_subchannel_initialize_dev() makes the ccw device lock be the subchannel
> lock. That means when one tries to get virtio config form within a cio
> interrupt context we deadlock, because we try to take a lock we already have.
> 
> That said, I don't think this limitation is by design (i.e. intended).
> Maybe Connie can help us with that question. AFAIK we have nothing
> documented regarding this (neither that can nor can't).
> 
> Obviously, there are multiple ways around this problem, and at the moment
> I can't tell which would be my preferred one.

Yes, it's also not difficult to tweak the virtio-balloon code to avoid that issue.
But if that's just an issue with ccw itself, I think it's better to tweak ccw and
remain virtio-balloon unchanged.

Best,
Wei