On 18.05.2017 11:05, Thomas Huth wrote: > On 18.05.2017 11:00, Christian Borntraeger wrote: >> On 05/18/2017 10:48 AM, David Hildenbrand wrote: >>> On 18.05.2017 03:55, Thomas Huth wrote: >>>> On 17.05.2017 18:49, David Hildenbrand wrote: >>>>> On 17.05.2017 17:35, Thomas Huth wrote: >>>>>> Currently we only present the plain z900 feature bits to the guest, >>>>>> but QEMU already emulates some additional features (but not all of >>>>>> the next CPU generation, so we can not use the next CPU level as >>>>>> default yet). Since newer Linux kernels are checking the feature bits >>>>>> and refuse to work if a required feature is missing, we should present >>>>>> as much of the supported features as possible when we are running >>>>>> with the default "qemu" CPU. >>>>>> This patch now adds the "stfle", "extended immediate" and "stckf" >>>>>> facility bits to the "qemu" CPU, which are already supported facilities. >>>>>> It is unfortunately still not enough to run e.g. recent Fedora kernels, >>>>>> but at least it's a first step into the right direction. >>>>>> >>>>> >>>>> Three things: >>>>> >>>>> 1. Should we care about backwards compatibility? I think so. This should >>>>> be fixed up using compat machine properties. (qemu model is a >>>>> migration-safe model and could e.g. be used in KVM setups, too). >>>> >>>> Theoretically, I agree, but do we really care about backwards >>>> compatibility at this point in time? All major distro kernels (except >>>> Debian, I think) currently do not work in QEMU, so there is currently >>>> not that much that can be migrated... >>>> And currently, the "qemu" CPU is the very same as the "z900" CPU, so you >>>> might also get along with simply using "-cpu z900" on the destination >>>> instead, I guess. >>> >>> If possible, I would like to avoid changing migration safe CPU model. >>> And I guess it shouldn't be too hard for now (unless we really change >>> the base model to e.g. a z9, then some more work might have to be done) >>> >>> I think for now, setting "stfle=off" on "s390-cpu-qemu" for compat >>> machines should do the trick. >>> >>>> >>>>> 2. I would recommend to not enable STFLE for now. Why? >>>>> >>>>> It is/was an indication that the system is running on a z9 (and >>>>> implicitly has the basic features). This was not only done because >>>>> people were lazy, but because this bit was implicitly connected to other >>>>> machine properties. >>>> >>>> Uh, that's ugly! >>>> >>>>> One popular example is the "DAT-enhancement facility 2". It introduced >>>>> the "LOAD PAGE TABLE ENTRY ADDRESS" instruction, but no facility bit was >>>>> introduced. SO there is no way to check if the instruction is available >>>>> and actually working. >>>> >>>> Does the Linux kernel use this instruction at all? I just grep'ed >>>> through the kernel sources and did not find it. If the Linux kernel does >>>> not use it, I think we should ignore this interdependency and just >>>> provide the STFLE feature bit to the guest - since recent Linux kernels >>>> depend on it. >>> >>> Yes, current linux doesn't use it, I don't remember if previous versions >>> did. Most likely not. The question is if they relied on the stfle==z9 >>> assumption. The STFLE facility really is special in that sense. >>> >>>> >>>>> Please note that we added a feature representation for this facility, >>>>> because this would allow us later on to at least model removal of such a >>>>> facility (if HW actually would drop it) on a CPU model level. >>>> >>>> What about STFLE bit 78, according to my version of the POP, it says: >>>> >>>> "The enhanced-DAT facility 2 is installed in the >>>> z/Architecture architectural mode." >>>> >>>> ? >>> >>> As Aurelien already mentioned, there seemed to be different ways to >>> enhance DAT :) enhanced-DAT facility 2 is 2GB page support. >>> >>>> >>>>> 3. This introduces some inconsistency. s390x/cpu_models.c:set_feature() >>>>> explicitly tests for such inconsistencies. >>>>> >>>>> So your QEMU CPU model would have a feature, but you would not be able >>>>> to run that model using QEMU when manually specifying it on the command >>>>> line. Especially, expanding the "qemu" model and feeding it back to QEMU >>>>> will fail. >>>> >>>> I've checked that I can also successfully disable the features again at >>>> the command line, using "-cpu qemu,eimm=false" for example, so not sure >>>> what exactly you're talking about here. Could you please elaborate? >>> >>> Assume libvirt/the user expands the CPU model name "qemu" via >>> "qmp-expand-cpu-model "qemu", you will get something like >>> >>> "z900-base,.....,stfle=on" >>> >>> If you feed that to QEMU using "-cpu z900-base,...,stfle=on", QEMU will >>> detect the inconsistency when setting the property and abort. However, >>> "-cpu qemu" will succeed. Please note that these checks actually make >>> sense for KVM: >>> >> >> Jason (now on cc) has a patch prepared for other reasons that disabled >> features >> for given machines. I kept the ESOP example in that patch. >> That would allow us to disable STFLE for old machines but enable it for 2.10 > [...] >> Maybe we should split that out and merge such a patch sooner than the >> (yet in development) other changes? > > Yes, that sounds like a good idea, I think we could use the same > mechanism here, too, so please split it out and submit it earlier! > > Thanks a lot, > Thomas >
I think this is useful but a different use case: What Christian/Jason have here is a way to fixup default models (e.g. z900, ZEC12...). This is necessary when introducing new features / movinf features from FULL into DEFAULT. We don't want to fixup default models but the s390x-cpu-qemu. -- Thanks, David