On 18.05.2017 11:14, David Hildenbrand wrote: > 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! >> > 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.
Looking at the functions again, I think you're right, David... it's similar at a first glance, but I'd need slightly different functions here. Ok, so never mind, I guess I have to do it in a different way here... Thomas