Re: [RFC PATCH v2 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR

Halil Pasic
2020-05-19T12:10:36Z

On Tue, 19 May 2020 13:36:06 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> On Tue, 19 May 2020 00:09:43 +0200
> Halil Pasic <pasic@linux.ibm.com> wrote:
>
> > On Mon, 18 May 2020 18:09:03 +0200
> > Cornelia Huck <cohuck@redhat.com> wrote:
>
> > > But taking a step back (and ignoring your series and the discussion,
> > > sorry about that):
> > >
> > > We need to do something (creating a local translation of the guest's
> > > channel program) that does not have any relation to the process in the
> > > architecture at all, but is only something that needs to be done
> > > because of what vfio-ccw is trying to do (issuing a channel program on
> > > behalf of another entity.)
> >
> > I violently disagree with this point. Looking at the whole vfio-ccw
> > device the translation is part of the execution of the channel program,
> > more specifically it fits in as prefetching. Thus it needs to happen
> > with the FC start bit set. Before FC start is set the subchannel is
> > not allowed to process (including look at) the channel program. At least
> > that is what I remember.
>
> I fear we really have to agree to disagree here.

So you say the subchannel is allowed to poke around before FC start is
set? I'm not sure what do you disagree with.

> The PoP describes how
> a SSCH etc. has to be done and what reaction to expect. It does not
> cover the 'SSCH on behalf of someone else' pattern: only what we can
> expect from that second SSCH, and what the guest can expect from us.

Yes. The PoP says that the guest can expect from us to not process the
submitted channel program if the subchannel is already busy. For this
whether the ccw device is fully emulated, or if it is a pass through
with 'SSCH on behalf' it does not matter. From the guest perspective
the 'SSCH on behalf' is just an implementation detail, right?

> In
> particular, the PoP does not specify anything about how a hypervisor is
> supposed to handle I/O from its guests (and why should it?)
>

Except that the exposed interface (towards the guest OS) needs to conform
to the pop. Including the point I cited.

> >
> > > Trying to sort that out by poking at actl
> > > and fctl bits does not seem like the best way; especially as keeping
> > > the bits up-to-date via STSCH is an exercise in futility.
> >
> > I disagree. A single subchannel is processing at most one channel
> > program at any given point in time. Or am I reading the PoP wrong?
>
> The hypervisor cannot know the exact status of the subchannel. It only
> knows the state of the subchannel at the time it issued its last STSCH.

You didn't respond to 'one cp at any given time is in PoP'. Do we agree
about that at least?

What can the hypervisor know about the host subchannel and its status
is a different issue. I would argue that it actually knows enough but
we need to get the very basics straight first.

But this point the hypervisor does not need to know about the exact
state of the host subchannel, it needs to know about the state of the
interface it is exposing (to the guest, or to the userspace).

> Anything else it needs to track is the hypervisor's business, and
> ideally, it should track that in its own control structures.

We agree on this. And this is what I'm asking for, and also what Eric
did. Although I don't agree with the details. He misused the actl bits.
According to him these were unused and thus the hypervisor's own control
structure. He himself stated that this may not be the best way to do it.

> (I know we
> muck around with the control bits today; but maybe that's not the best
> idea.)
>

What control bits do you mean?

> >
> > >
> > > What about the following (and yes, I had suggested something vaguely in
> > > that direction before):
> > >
> > > - Detach the cp from the subchannel (or better, remove the 1:1
> > > relationship). By that I mean building the cp as a separately
> > > allocated structure (maybe embedding a kref, but that might not be
> > > needed), and appending it to a list after SSCH with cc=0. Discard it
> > > if cc!=0.
> > > - Remove the CP_PENDING state. The state is either IDLE after any
> > > successful SSCH/HSCH/CSCH, or a new state in that case. But no
> > > special state for SSCH.
> > > - A successful CSCH removes the first queued request, if any.
> > > - A final interrupt removes the first queued request, if any.
> > >
> > > Thoughts?
> > >
> >
> > See above. IMHO the second SSCH is to be rejected by QEMU. I've
> > explained this in more detail in my previous mail.
>
> I don't think we should rely on whatever QEMU is or isn't doing. We
> should not get all tangled up if userspace is doing weird stuff.

I agree 100%. And I was a vocal advocate of this all the time. We
need to avoid undefined behavior in the kernel regardless of what
userspace is doing.

But we can (and should) have expectations towards the userspace,
and refuse to do any further work, if we detect that userspace is
misbehaving.

Regards,
Halil

>