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

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

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.

> 
> > 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.)

> > 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.

Regards,
Halil