Messages in this thread | | | Date | Tue, 20 May 2014 12:10:23 +0200 | From | Alexander Graf <> | Subject | Re: [Qemu-devel] [PATCH v1 RFC 6/6] KVM: s390: add cpu model support |
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On 20.05.14 12:02, Michael Mueller wrote: > On Mon, 19 May 2014 22:14:00 +0200 > Alexander Graf <agraf@suse.de> wrote: > >> On 19.05.14 19:03, Michael Mueller wrote: >>> On Mon, 19 May 2014 16:49:28 +0200 >>> Alexander Graf <agraf@suse.de> wrote: >>>
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>>>>> What user and thus also user space wants depends on other factors: >>>>> >>>>> 1. reliability >>>>> 2. performance >>>>> 3. availability >>>>> >>>>> It's not features, that's what programmers want. >>>>> >>>>> That's why I have designed the model and migration capability around the hardware >>>>> and not around the software features and don't allow them to be enabled currently >>>>> together. >>>>> >>>>> A software feature is a nice add on that is helpful for evaluation or development >>>>> purpose. There is few space for it on productions systems. >>>>> >>>>> One option that I currently see to make software implemented facility migration >>>>> capable is to calculate some kind of hash value derived from the full set of >>>>> active software facilities. That value can be compared with pre-calculated >>>>> values also stored in the supported model table of qemu. This value could be >>>>> seen like a virtual model extension that has to match like the model name. >>>>> >>>>> But I have said it elsewhere already, a soft facility should be an exception and >>>>> not the rule. >>>>> >>>>>>>> So all we need is a list of "features the guest sees available" which is >>>>>>>> the same as "features user space wants the guest to see" which then gets >>>>>>>> masked through "features the host can do in hardware". >>>>>>>> >>>>>>>> For emulation we can just check on the global feature availability on >>>>>>>> whether we should emulate them or not. >>>>>>>> >>>>>>>>>> Also, if user space wants to make sure that its feature list is actually >>>>>>>>>> workable on the host kernel, it needs to set and get the features again >>>>>>>>>> and then compare that with the ones it set? That's different from x86's >>>>>>>>>> cpuid implementation but probably workable. >>>>>>>>> User space will probe what facilities are available and match them with the predefined >>>>>>>>> cpu model set. Only those models which use a partial or full subset of the hard/host >>>>>>>>> facility list are selectable. >>>>>>>> Why? >>>>>>> If a host does not offer the features required for a model it is not able to >>>>>>> run efficiently. >>>>>>> >>>>>>>> Please take a look at how x86 does cpuid masking :). >>>>>>>> >>>>>>>> In fact, I'm not 100% convinced that it's a good idea to link cpuid / >>>>>>>> feature list exposure to the guest and actual feature implementation >>>>>>>> inside the guest together. On POWER there is a patch set pending that >>>>>>>> implements these two things separately - admittedly mostly because >>>>>>>> hardware sucks and we can't change the PVR. >>>>>>> That is maybe the big difference with s390. The cpuid in the S390 case is not >>>>>>> directly comparable with the processor version register of POWER. >>>>>>> >>>>>>> In the S390 world we have a well defined CPU model room spanned by the machine >>>>>>> type and its GA count. Thus we can define a bijective mapping between >>>>>>> (type, ga) <-> (cpuid, ibc, facility set). From type and ga we form the model >>>>>>> name which BTW is meaningful also for a human user. >>>>>> Same thing as POWER. >>>>>> >>>>>>> By means of this name, a management interface (libvirt) will draw decisions if >>>>>>> migration to a remote hypervisor is a good idea or not. For that it just needs >>>>>>> to compare if the current model of the guest on the source hypervisor >>>>>>> ("query-cpu-model"), is contained in the supported model list of the target >>>>>>> hypervisor ("query-cpu-definitions"). >>>>>> I don't think this works, since QEMU should always return all the cpu >>>>>> definitions it's aware of on query-cpu-definitions, not just the ones >>>>>> that it thinks may be compatible with the host at a random point in time. >>>>> It does not return model names that it thinks they are compatible at some point >>>>> in time. In s390 mode, it returns all definitions (CPU models) that a given host >>>>> system is capable to run. Together with the CPU model run by the guest, some upper >>>>> management interface knows if the hypervisor supports the required CPU model and >>>>> uses a guest definition with the same CPU model on the target hypervisor. >>>>> >>>>> The information for that is taken from the model table which QEMU builds up during >>>>> startup time. This list limits the command line selectable CPU models as well. >>>> This makes s390 derive from the way x86 handles things. NAK. >>> One second, that goes a little fast here :-). x86 returns a list they support which happens to >>> be the full list they define and s390 does logically the same because we know that certain >>> models are not supported due to probing. BTW that happens only if you run Qemu on back >>> level hardware and that is perfectly correct. >> It's not what other architectures do and I'd hate to see s390 deviate >> just because. > Only these four architectures implement the query and they all differ a little... > > target-arm/helper.c:CpuDefinitionInfoList *arch_query_cpu_definitions(Error **errp) > target-i386/cpu.c:CpuDefinitionInfoList *arch_query_cpu_definitions(Error **errp) > target-ppc/translate_init.c:CpuDefinitionInfoList *arch_query_cpu_definitions(Error **errp) > target-s390x/cpu.c:CpuDefinitionInfoList *arch_query_cpu_definitions(Error **errp) > > arm walks through a list of all ARM CPU types > list = object_class_get_list(TYPE_ARM_CPU, false); > and returns the CpuDefinitionInfoList derived from that one to one > > i386 loops over the static builtin_x86_defs[] array to retrieve the model names, > they don't even use the CPU class model as source > > ppc walks through a list of all POWER CPU types > list = object_class_get_list(TYPE_POWERPC_CPU, false); > and then extends the produced list by all defined aliases > > and s390x finally also walks through the defined S390 CPU types > list = object_class_get_list(TYPE_S390_CPU, false); > but drops those which are not usable (!is_active) > Just consider them as not defined. I actually would undefine > them if I knew how. > > Also the commands comment says "list of supported virtual CPU definitions" and the s390 > list contains all supported models, that's no contradiction.
So IMHO we can either
a) change the definition of query_cpu_definitions to only return CPUs that are executable with KVM on a given machine (probably a bad idea) or b) return not only the CPU type, but also a hint whether it's available with KVM or c) add a parameter to query_cpu_definitions to say "only return KVM runnable CPUs" or d) introduce a new query_kvm_cpu_definitions qmp command
> > ## > # @query-cpu-definitions: > # > # Return a list of supported virtual CPU definitions > # > # Returns: a list of CpuDefInfo > >>> The migration compatibility test is pretty much ARCH dependent. I looked into the >>> libvirt implementation and as one can see every architecture has its own implementation >>> there (libvirt/src/cpu/cpu_<arch>.c). >> So here's my question again. How does x86 evaluate whether a target >> machine is compatible with a source machine? > Will again look into that during the afternoon...
Yes, please. Someone else must have solved this before :).
Alex
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