Re: [Qemu-devel] [PATCH v2 2/5] vfio-ccw: concurrent I/O handling

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 28 Jan 2019 20:30:00 +0100
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Mon, 28 Jan 2019 18:13:55 +0100
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > On Fri, 25 Jan 2019 17:04:04 +0100
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> >   
> > > Do we expect userspace/QEMU to fence the bad scenarios as tries to do
> > > today, or is this supposed to change to hardware should sort out
> > > requests whenever possible.  
> > 
> > Does my other mail answer that?  
> 
> Sorry, I can't find the answer in your other (Date: Mon, 28 Jan 2019
> 17:59:10 +0100, Message-Id: <20190128175910.5d9677e7@oc2783563651>) mail.
> AFAIU that mail talks abut the kernel and not about the userspace.
> 
> I guess the answer is we don't expect changes to userspace, so we do
> expect userspace to fence bad scenarios.

Then, I really have no idea what you are aiming at with your comment :(

> 
> >   
> > > The problem I see with the let the hardware sort it out is that, for
> > > that to work, we need to juggle multiple translations simultaneously
> > > (or am I wrong?). Doing that does not appear particularly simple to
> > > me.  
> > 
> > None in the first stage, at most two in the second stage, I guess.
> >   
> 
> Expected benefit of the second stage over the first stage? (I see none.)

Making something possible that is allowed by the architecture. Not
really important, though.

> 
> > > Furthermore we would go through all that hassle knowingly that the
> > > sole reason is working around bugs. We still expect our Linux guests
> > > serializing it's ssch() stuff as it does today. Thus I would except
> > > this code not getting the love nor the coverage that would guard
> > > against bugs in that code.  
> > 
> > So, we should have test code for that? (Any IBM-internal channel I/O
> > exercisers that may help?)
> >  
> 
> None that I'm aware of. Anyone else? 
> 
> But the point I was trying to make is the following: I prefer keeping
> the handling for the case "ssch()'s on top of each other" as trivial as
> possible. (E.g. bail out if CP_PENDING without doing any translation.)
>  
> > We should not rely on the guest being sane, although Linux probably is
> > in that respect.
> >   
> 
> I agree 100%: we should not rely on either guest or userspace emulator
> being sane. But IMHO we should handle insanity with the least possible
> investment.

We probably disagree what the least possible investment is.