linux-kernel - Re: [PATCH 1/1] vfio-ccw: Enable transparent CCW IPL from DASD

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Message-ID: <20200423171103.497dcd02.cohuck@redhat.com>
Date:   Thu, 23 Apr 2020 17:11:03 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Jared Rossi <jrossi@...ux.ibm.com>,
        Eric Farman <farman@...ux.ibm.com>, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] vfio-ccw: Enable transparent CCW IPL from DASD

On Thu, 23 Apr 2020 15:56:20 +0200
Halil Pasic <pasic@...ux.ibm.com> wrote:

> On Fri, 17 Apr 2020 14:29:39 -0400
> Jared Rossi <jrossi@...ux.ibm.com> wrote:
> 
> > Remove the explicit prefetch check when using vfio-ccw devices.
> > This check is not needed as all Linux channel programs are intended
> > to use prefetch and will be executed in the same way regardless.  
> 
> Hm. This is a guest thing or? So you basically say, it is OK to do
> this, because you know that the guest is gonna be Linux and that it
> the channel program is intended to use prefetch -- but the ORB supplied
> by the guest that designates the channel program happens to state the
> opposite.
> 
> Or am I missing something?

I see this as a kind of architecture compliance/ease of administration
tradeoff, as we none of the guests we currently support uses something
that breaks with prefetching outside of IPL (which has a different
workaround).

One thing that still concerns me a bit is debuggability if a future
guest indeed does want to dynamically rewrite a channel program: the
guest thinks it instructed the device to not prefetch, and then
suddenly things do not work as expected. We can log when a guest
submits an orb without prefetch set, but we can't find out if the guest
actually does something that relies on non-prefetch.

The only correct way to handle this would be to actually implement
non-prefetch processing, where I would not really know where to even
start -- and then we'd only have synthetic test cases, for now. None of
the options are pleasant :(