Re: [PATCH 0/3] vfio-ccw: support hsch/csch (kernel part)

[Halil Pasic <pasic@linux.ibm.com>]

On Fri, 7 Dec 2018 11:05:29 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

[..]
> > To clarify my concern let me quote from the PoP
> > (SA22-7832-10 page 14-9):
> > 
> > """
> > If a device presents unsolicited status while the
> > associated subchannel is disabled, that status is
> > discarded by the channel subsystem without
> > generating an I/O-interruption condition. How-
> > ever, if the status presented contains unit check,
> > the channel subsystem issues the clear signal for
> > the associated subchannel and does not gener-
> > ate an I/O-interruption condition. This should be
> > taken into account when the program uses MOD-
> > IFY SUBCHANNEL to enable a subchannel. For
> > example, the medium on the associated device
> > that was present when the subchannel became
> > disabled may have been replaced, and, there-
> > fore, the program should verify the integrity of
> > that medium.
> > """
> 
> Hm, so is your concern that we might have a status (unit check) if we
> have an enabled subchannel that might not be present if the subchannel
> had been disabled all the time? Is that a problem in practice?
> 

No idea if it is a problem in practice.

> > > > 
> > > > I think Jason has discovered some problems related to this while
> > > > doing his DASD IPL with vfio-ccw work, but I don't quite remember
> > > > any more.  
> > > 
> > > cc:ing Jason, in case he remembers :)
> 
> Like in that case. Couldn't a unit check status also arrive just when
> the subchannel has been enabled, and the code therefore has to deal
> with it anyway?
> 

I assumed that programming note is there for a reason. Of course if
it can not been proven it ain't cheating. I don't remember exactly this
interacts with the rest of the architecture. In fact I asked my question,
because my feeling was that tying the virtual an the backing subchannel
together is simpler, than proving that we are fine without doing it.


> > >   
> > > > IMHO it may be possible to emulate enable/disable, but it seems way
> > > > more error prone and complicated, than letting the guest
> > > > enable/disable the host subchannel.
> > > > 
> > > > I have no idea what was the reason for going with the initial design.
> > > > I would appreciate any hints or explanations, but I'm well aware
> > > > that it was a long time ago.  
> > > 
> > > I don't really remember either, and any non-public mails from that time
> > > are inaccessible to me :(
> > > 
> > > It *might* be an artifact of the original design (which operated at the
> > > ccw_device rather than the subchannel level), though.
> > >   
> > 
> > Interesting.
> > 
> > > > > Parameters (like for channel measurements) are a different game.
> > > > > It is something we should look into, but it will need a different
> > > > > region.    
> > > > 
> > > > Yes emulation only channel measurements seem even less likely than
> > > > proper enable/disable. And 'that would need a different' region
> > > > helps me understanding the scope of async_cmd_region. Maybe we
> > > > should reconsider the comment '+ * @cmd_region: MMIO region for
> > > > asynchronous I/O commands other than START'.  
> > > 
> > > What do you think is wrong with that comment?
> > >   
> > 
> > Well msch is also an async I/O command other than START. If msch does not
> > belong here but needs it's own region, then this description seems too
> > generic.
> 
> Why do you consider msch to be async? ssch, hsch, csch all have the
> potential to cause the execution of an asynchronous (start/halt/clear)
> function, while msch just (possibly) modifies the subchannel and is
> done.
>

Right, my bad. Got confused by my Z channel io is async superstition. I
did not quite understand what async means in this context.

Regards,
Halil