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

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 6 Dec 2018 10:26:12 -0500
Farhan Ali <alifm@linux.ibm.com> wrote:

> On 12/06/2018 09:39 AM, Cornelia Huck wrote:
> > On Wed, 5 Dec 2018 13:34:11 -0500
> > Farhan Ali <alifm@linux.ibm.com> wrote:
> >   
> >> On 12/05/2018 07:54 AM, Cornelia Huck wrote:  
> >>>> 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.
> >>>
> >>> For your example, it might be best if a hsch is always accepted and
> >>> send on towards the hardware. Probably best to reflect back -EAGAIN if
> >>> we're currently processing another instruction from another vcpu, so
> >>> that the user space caller can retry. 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.
> >>>
> >>> 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.
> >>>      
> >>
> >> If we have a ssch and a csch immediately afterwards from userspace, will
> >> we end up issuing csch first and then ssch to the hardware?
> >>
> >> If I understand correctly, the ccw translation as part of the ssch can
> >> be a slow operation so it might be possible we issue the csch first?
> >> In that case we won't actually clear the original start function as
> >> intended.  
> > 
> > When we start processing the ssch request (translation and so on), we
> > set the state to BUSY. This means that any csch request will get a
> > -EBUSY, no overtaking possible. (I think maybe I'll need to check what
> > this series looks like if I rebase it on top of Pierre's rework, as he
> > did some changes in the state machine.)  
> 
> I think you meant the state is set to BOXED? otherwise the patch 3 says 
> if state is BUSY and CLEAR event request comes in, we issue the clear 
> instruction, no?

That's what I meant with "need to rebase" :) The BOXED state is gone; I
just had not rebased on top of it. There's more changes in the state
machine; if we are on the same page as to what should happen, I can
start massaging the patches.