Re: [PATCH v3 1/1] vfio-ccw: Prevent quiesce function going into an infinite loop

Halil Pasic
2019-04-24T10:21:10Z

On Tue, 23 Apr 2019 15:41:34 -0400
Farhan Ali <alifm@linux.ibm.com> wrote:

>
>
> On 04/23/2019 01:42 PM, Halil Pasic wrote:
> > One thing I'm confused about is, that we don't seem to prevent
> > new I/O being submitted. That is we could still loop indefinitely
> > if we get new IO after the 'kill I/O on the subchannel' is done but
> > before the msch() with the disable is issued.
>
> So the quiesce function will be called in the remove, release functions
> and also in the mdev reset callback via an ioctl VFIO_DEVICE_RESET.
>
> Now the release function is invoked in cases when we hot unplug the
> device or the guest is gone (or anything that will close the vfio mdev
> file descriptor, I believe). In such scenarios it's really the userspace
> which is asking to release the device. Similar for remove, where the
> user has to explicitly write to the remove file for the mdev to invoke
> it. Under normal conditions no sane userspace should be doing
> release/remove while there are still on going I/Os :)

So you say userspace has to take care of it. Is this documented
somewhere? Does QEMU actually take care of it?

>
> Me and Conny had some discussion on this in v1 of this patch:
> https://marc.info/?l=kvm&m=155437117823248&w=2
>
> >
> > The 'flush all I/O' parts in the commit message and in the code make
> > this even more confusing.
>
> Maybe...if it's too confusing it could be fixed, but IMHO I don't think
> it's a dealbreaker. If anyone else thinks otherwise, I can go ahead and
> change it.
>

I responded to Connie. No it is not a dealbreaker. But I prefer not
disseminating false information.

[..]

Regarding the rest of the points from her, I responded to Connie's
follow up as well.

Cheers,
Halil