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

[Farhan Ali]

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.

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?