Re: [Qemu-devel] [PATCH v2 2/5] vfio-ccw: concurrent I/O handling

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 28 Jan 2019 16:48:10 -0500
Eric Farman <farman@linux.ibm.com> wrote:

> On 01/28/2019 02:15 PM, Halil Pasic wrote:
> > On Mon, 28 Jan 2019 18:09:48 +0100
> > Cornelia Huck <cohuck@redhat.com> wrote:

> I guess if
> > the ssch() returns with non cc == 0 the CP_PENDING ---IRQ---> IDLE
> > transition
> > won't take place. And I guess the IRQ is a final one.  
> 
> Yes this is the one point I hadn't seen explicitly stated.  We shouldn't 
> remain in state=BUSY if the ssch got cc!=0, and probably return to IDLE 
> when processing the failure.  In Connie's response (Mon, 28 Jan 2019 
> 18:24:24 +0100) to my note, she expressed some agreement to that.

Yes, I think that's what should happen.


> >> state for I/O)
> >> (normal ssch)
> >>
> >> BUSY --- IO_REQ ---> return -EAGAIN, stay in BUSY
> >> (user space is supposed to retry, as we'll eventually progress from
> >> BUSY)
> >>
> >> CP_PENDING --- IO_REQ ---> return -EBUSY, stay in CP_PENDING
> >> (user space is supposed to map this to the appropriate cc for the guest)  
> > 
> >  From this it seems you don't intend to issue the second  requested ssch()
> > any more (and don't want to do any translation). Is that right? (If it
> > is, that what I was asking for for a while, but then it's a pity for the
> > retries.)
> >   
> >>
> >> IDLE --- ASYNC_REQ ---> IDLE
> >> (user space is welcome to do anything else right away)  
> > 
> > Your idea is to not issue a requested hsch() if we think we are IDLE
> > it seems. Do I understand this right? We would end up with a different
> > semantic for hsch()/and csch() (compared to PoP) in the guest with this
> > (AFAICT).
> >   
> >>
> >> BUSY --- ASYNC_REQ ---> return -EAGAIN, stay in BUSY
> >> (user space is supposed to retry, as above)
> >>
> >> CP_PENDING --- ASYNC_REQ ---> return success, stay in CP_PENDING
> >> (the interrupt will get us out of CP_PENDING eventually)  
> > 
> > Issue (c|h)sch() is an action that is done on this internal
> > transition (within CP_PENDING).  
> 
> These three do read like CSCH/HSCH are subject to the same rules as 
> SSCH, when in fact they would be (among other reasons) issued to clean 
> up a lost interrupt from a previous SSCH.  So maybe return -EAGAIN on 
> state=BUSY (don't race ourselves with the start), but issue to hardware 
> if CP_PENDING.

I think there are some devices which require a certain hsch/csch
sequence during device bringup, so it's not just cleaning up after a
ssch. Therefore, we should always try to do the requested hsch/csch,
unless things like "we're in the process of translating a cp, and can't
deal with another request right now" prevent it.

> 
> If we get an async request when state=IDLE, then maybe just issue it for 
> fun, as if it were an SSCH?

For fun, but mainly because the guest wants it :)