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

Cornelia Huck
2019-04-24T07:09:15Z

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 :)
>
> 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 think it's fine -- I wasn't confused.

>
> >
> > Another thing that I don't quite understand is injecting interrupts
> > into QEMU for stuff that is actually not guest initiated.
>
> As mentioned above under normal conditions we shouldn't be doing
> quiesce. But wouldn't those interrupts just be unsolicited interrupts
> for the guest?

Yes, you simply cannot keep an enabled subchannel from getting status
pending with unsolicited status.

>
> >
> > Furthermore I find how cio_cancel_halt_clear() quite confusing. We

Well, that's a problem (if any) with the common I/O layer and beyond
the scope of this patch.

> > TL;DR:
> >
> > I welcome this batch (with an r-b) but I would like the commit message

So, what does this sentence mean? Confused.

> > and the comment changed so that the misleading 'flush all I/O in the
> > workqueue'.
> >
> > I think 'vfio-ccw: fix cio_cancel_halt_clear() usage' would reflect the
> > content of this patch better, because reasoning about the upper limit,
> > and what happens if this upper limit is hit is not what this patch is
> > about. It is about a client code bug that rendered iretry ineffective.
> >
>
> I politely disagree with the change in subject line. I think the current
> subject line describe what we are trying to prevent with this patch. But
> again if anyone else feels otherwise, I will go ahead and change :)

No, I agree that the subject line is completely fine.