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