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

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Date:   Fri, 24 Apr 2020 15:02:40 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Cornelia Huck <cohuck@...hat.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 Mon, 20 Apr 2020 14:13:17 +0200
Cornelia Huck <cohuck@...hat.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... my understanding is that we have the reasonable expectation that
> our guests will always issue channel programs that work fine with
> prefetch even if the bit is not set in the orb (including CCW IPL, in
> the way it is implemented in the s390-ccw QEMU bios), and therefore
> this patch is just making things less complicated.

AFAIR the problem is not s390-ccw QEMU bios. We could easily fix that.
The practical problem is some channel programs generated by zipl that
simply fail to set the bit (although they could). That is a perfectly
legit thing to do, because the prefetch bit was originally about
performance.

Sorry I missed your mail, so complained about the same
issues.

Regards,
Halil