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

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

On Wed, 5 Dec 2018 13:54:02 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> On Tue, 4 Dec 2018 16:02:36 +0100
> Halil Pasic <pasic@linux.ibm.com> wrote:
> 
> > On Tue, 4 Dec 2018 14:11:30 +0100
> > Cornelia Huck <cohuck@redhat.com> wrote:
> > 
> > > On Tue, 4 Dec 2018 13:38:10 +0100
> > > Halil Pasic <pasic@linux.ibm.com> wrote:
> > >   
> > > > On Thu, 22 Nov 2018 17:54:29 +0100
> > > > Cornelia Huck <cohuck@redhat.com> wrote:
> > > >   
> > > > > [This is the Linux kernel part, git tree is available at
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/kvms390/vfio-ccw.git
> > > > > vfio-ccw-caps
> > > > > 
> > > > > The companion QEMU patches are available at
> > > > > https://github.com/cohuck/qemu vfio-ccw-caps]
> > > > > 
> > > > > Currently, vfio-ccw only relays START SUBCHANNEL requests to
> > > > > the real device. This tends to work well for the most common
> > > > > 'good path' scenarios; however, as we emulate {HALT,CLEAR}
> > > > > SUBCHANNEL in QEMU, things like clearing pending requests at
> > > > > the device is currently not supported. This may be a problem
> > > > > for e.g. error recovery.    
> > > > 
> > > > I'm wondering: what about MODIFY SUBCHANNEL? Do we plan to add
> > > > MSCH as well or is it supposed to remain 'userspace emulated'?
> > > > AFAIR MSCH may have an effect on error recovery as well.  
> > > 
> > > I think that would require a deeper change, as we have the
> > > requirement to enable the subchannel before handing it to
> > > userspace. IOW, the guest does not cause the subchannel to be
> > > enabled/disabled, but the host does. 
> > 
> > My point is, when the subchannel is disabled, 'firmware' is
> > responsible for suppressing interrupts and error conditions, and
> > also for doing the appropriate recovery procedure, so to say under
> > the hood.
> 
> I don't think there's actually much of a 'recovery' possible at a
> subchannel level (other than 'have you tried turning it off and on
> again?'); the interesting stuff is all at the device-specific level.
> 

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.
"""

> > 
> > 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 :)
> 
> > 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.

> > > > BTW I would like to have the concurrency discussion sorted out
> > > > before I proceed with my review, because reviewing the stuff
> > > > without a fair idea of what exactly are we trying to achieve
> > > > would yield poor results.  
> > > 
> > > I'm not sure what is unclear about what we're trying to achieve
> > > (enable the guest to issue halt/clear on real hardware)?  
> > 
> > Yeah, that is perfectly clear, but it ain't the complete story. E.g.
> > are subsequent commands blocking until the preceding command finishes
> > is part of the interface. And what is good implementation depends on
> > the answer. What I mean, I first need to understand how things are
> > supposed to work (together) so I can double check that against the
> > implementation. Otherwise all I can do is nitpicking.
> > 
> > To get more tangible: we are in the middle of processing an SSCH
> > request (e.g. doing the translation) when a HSCH comes in. What
> > should happen? Should we start processing HSCH after he instruction
> > part of SSCH is done -- which currently includes translation? Or
> > should we -EBUSY? Or do we abort START related activities and do the
> > HALT stuff?
> 
> I think most of the sorting-out-the-operations stuff should be done by
> the hardware itself, and we should not really try to enforce anything
> special in our vfio code.
> 

Sounds very reasonable to me. Does this mean you are against Pierre's
'[PATCH v3 6/6] vfio: ccw: serialize the write system calls' as it does
not let HW sort out stuff, but enforces sequencing?


> For your example, it might be best if a hsch is always accepted and
> send on towards the hardware.

Nod.

> Probably best to reflect back -EAGAIN if
> we're currently processing another instruction from another vcpu, so
> that the user space caller can retry.

Hm, not sure how this works together with your previous sentence.

> Same for ssch, if another ssch is
> already being processed. We *could* reflect cc 2 if the fctl bit is
> already set, but that won't do for csch, so it is probably best to have
> the hardware figure that out in any case.
> 

We just need to be careful about avoiding races if we let hw sort out
things. If an ssch is issued with the start function pending the correct
response is cc 2. 

> If I read the code correctly, we currently reflect -EBUSY and not
> -EAGAIN if we get a ssch request while already processing another one.
> QEMU hands that back to the guest as a cc 2, which is not 100% correct.
> In practice, we don't see this with Linux guests due to locking.
> 

Nod, does not happen because of BQL. We currently do the user-space
counterpart of vfio_ccw_mdev_write() in BQL context or (i.e. we hold
BQL until translation is done and our host ssch() comes back)?

I think -EBUSY is the correct response for ssch while start pending set.
I think we set start pending in QEMU before we issue 'start command/io
request' to the kernel. I don't think -EAGAIN is a good idea. AFAIU we
would expect user-space to loop on -EAGAIN e.g. at least until the
processing of a 'start command' is done and the (fist) ssch by the host
is issued. And then what?  Translate the second channel program issue
the second ssch in the host and probably get a non-zero cc? Or return
-EBUSY? Or keep returning -EAGAIN?

> > > But yes, we need to sort out that concurrency thing; I'm currently
> > > unsure if the core should do some things as well or if it's more of
> > > a vendor-driver thing.
> > >   
> > 
> > By core you mean vfio-mdev core? If yes, I think it is a
> > vendor-driver thing: limiting concurrency for all vfio-mdev  does
> > not make sense IMHO.
> 
> Also generic vfio. But I'm still unclear which guarantees we have. I
> suspect none; I'm wondering whether other vfio devices might have
> issues as well.
> 

My intuition says this is something left to the devices.

Regards,
Halil