On Thu, 23 Apr 2020 16:25:39 -0400 Eric Farman <farman@xxxxxxxxxxxxx> wrote: > > > On 4/23/20 11:11 AM, Cornelia Huck wrote: > > 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).> 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
