Re: [virtio-dev] RE: [PATCH v1 0/2] Virtio: fix some vq allocation issues

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 2 Jan 2019 16:59:19 +0100
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Wed, 2 Jan 2019 14:23:38 +0100
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Wed, 2 Jan 2019 10:53:14 +0100
> > Cornelia Huck <cohuck@redhat.com> wrote:
> >   
> > > On Tue, 1 Jan 2019 00:40:19 +0100
> > > Halil Pasic <pasic@linux.ibm.com> wrote:  

> > > > AFAICT tweaking the balloon code may be simpler than tweaking the
> > > > virtio-ccw (transport code). ccw_io_helper() relies on getting
> > > > an interrupt when the issued IO is done. If virtio-ccw is buggy, it
> > > > needs to be fixed, but I'm not sure it is.    
> > > 
> > > I would not call virtio-ccw buggy, but it has some constraints that
> > > virtio-pci apparently doesn't have (and which did not show up so far;
> > > e.g. virtio-blk schedules a work item on config change, so there's no
> > > deadlock there.)
> > > 
> > > One way to get out of that constraint (don't interact with the config
> > > space directly in the config changed handler) would be to schedule a
> > > work item in virtio-ccw that calls virtio_config_changed() for the
> > > device. My understanding is that delaying the notification to a work
> > > queue would be fine.  
> > 
> > Unfortunately, calling virtio_config_changed() from a work item is not
> > enough: That function takes the config_lock, and the virtio-ccw code to
> > get the config both needs to allocate some memory and call schedule :/
> > 
> > The best option really seems to be
> > - have virtio-balloon move the handling of the config change onto a
> >   workqueue or something like that, and
> > - document that you cannot read/write the virtio config space from an
> >   atomic context
> > 
> > Unless someone has a better idea?
> >   
> 
> I wonder, would making config_lock a mutex suffice?

Unless I'm mistaken, you can't take a mutex in an interrupt path.

> I've already hinted that a virtio-balloon side fix is probably the
> simpler variant. 

Yes, I think so as well.

> I agree, let's fix the regression first, and think about wether to teach
> virtio-ccw to allow config manipulation from virtio_config_changed() or
> not later.

Or whether we can tweak the virtio code instead. But I agree, let's get
things working again first.