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Re: [qemu-s390x] [Qemu-devel] [PATCH 0/3] vfio-ccw: support hsch/csch (k

From: Cornelia Huck
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH 0/3] vfio-ccw: support hsch/csch (kernel part)
Date: Fri, 7 Dec 2018 10:34:41 +0100

On Thu, 6 Dec 2018 12:50:50 -0500
Farhan Ali <address@hidden> wrote:

> On 12/06/2018 11:21 AM, Cornelia Huck wrote:
> > On Thu, 6 Dec 2018 10:26:12 -0500
> > Farhan Ali <address@hidden> wrote:
> >   
> >> On 12/06/2018 09:39 AM, Cornelia Huck wrote:  
> >>> On Wed, 5 Dec 2018 13:34:11 -0500
> >>> Farhan Ali <address@hidden> 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.
> > 
> >   
> Sorry maybe I missed it, but are you referring to Pierre's latest 
> cleanup patches? I don't see him removing the BOXED state.

The "remove BOXED state" patch is currently on my vfio-ccw-staging
branch. (That reminds me, will need to move it to my vfio-ccw branch
and possibly send a pull request. I had hoped to collect more patches
for the next release...)

> I think returning -EAGAIN and asking the userspace to retry the 
> operation sounds reasonable to me.
> But how do we handle the issue of protecting the cmd_region from 
> simultaneous hsch and csch calls? Do we agree on Pierre's method of 
> making write calls mutually exclusive?

That's in his patch series, right? I did not yet have time to look at

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