Messages in this thread | | | Date | Tue, 28 Apr 2020 20:38:53 -0400 | From | Jared Rossi <> | Subject | Re: [PATCH 1/1] vfio-ccw: Enable transparent CCW IPL from DASD |
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On 2020-04-24 08:50, Halil Pasic wrote: > 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. >
From what I previously did with the tracing, I don't think that there is a practical way to determine if a cp is actually doing something that relies on non-prefetch. It seems we would need to examine the CCWs to find reads and also validate the addresses those CCWs access to check if there is a conflict. Probably this is too much overhead considering that we expect it to be a rare occurrence?
Is it too simplistic to print a kernel warning stating that an ORB did not have the p-bit set, but it is being prefetched anyway?
Regards, Jared Rossi
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