On Thu, 23 Apr 2020 15:56:20 +0200 Halil Pasic <pasic@xxxxxxxxxxxxx> wrote: > On Fri, 17 Apr 2020 14:29:39 -0400 > Jared Rossi <jrossi@xxxxxxxxxxxxx> 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 :(
