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Date:   Fri, 24 Apr 2020 14:50:07 +0200
From:   Halil Pasic <pasic@linux.ibm.com>
To:     Eric Farman <farman@linux.ibm.com>
Cc:     Cornelia Huck <cohuck@redhat.com>,
        Jared Rossi <jrossi@linux.ibm.com>, linux-s390@vger.kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/1] vfio-ccw: Enable transparent CCW IPL from DASD
Message-ID: <20200424145007.75101d10.pasic@linux.ibm.com>
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        <20200423155620.493cb7cb.pasic@linux.ibm.com>
        <20200423171103.497dcd02.cohuck@redhat.com>
        <b6dc3d32-3e84-4ce1-59a2-d5de99716027@linux.ibm.com>
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On Thu, 23 Apr 2020 16:25:39 -0400
Eric Farman <farman@linux.ibm.com> wrote:

> 
> 
> On 4/23/20 11:11 AM, Cornelia Huck wrote:
> > On Thu, 23 Apr 2020 15:56:20 +0200
> > Halil Pasic <pasic@linux.ibm.com> wrote:
> > 
> >> On Fri, 17 Apr 2020 14:29:39 -0400
> >> Jared Rossi <jrossi@linux.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).>

And that workaround AFAIR makes sure that we don't issue a CP that is
self-modifying or otherwise reliant on non-prefetch. So any time we see
a self-modifying program we know, we have an incompatible setup.

In any case I believe the commit message is inadequate, as it does not
reflect about the risks.

> > One thing that still concerns me a bit is debuggability if a future
> > guest indeed does want to dynamically rewrite a channel program: the
> 
> +1 for some debuggability, just in general
> 
> > 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.
> 
> Without going too far down a non-prefetch rabbit-hole, can we use the
> cpa_within_range logic to see if the address of the CCW being fetched
> exists as the CDA of an earlier (non-TIC) CCW in the chain we're
> processing, and tracing/logging/messaging something about a possible
> conflict?
> 
> (Jared, you did some level of this tracing with our real/synthetic tests
> some time ago.  Any chance something of it could be polished and made
> useful, without being overly heavy on the mainline path?)
> 

Back then I believe I made a proposal on how this logic could look like.
I think all we need is checking for self rewrites (ccw reads to the
addresses that comprise the  complete original channel program), and for
status-modifier 'skips'. The latter could be easily done by putting some
sort of poison at the end of the detected channel program segments.

> > 
> > 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 :(
> > 
> 

I don't think implementing non-prefetch processing is possible with
vfio-ccw. 

Regards,
Halil