Re: Default CPU models on s390x and ppc64

[Christian Borntraeger]

On 17.10.19 17:18, David Hildenbrand wrote:
> On 17.10.19 17:16, Jiri Denemark wrote:
>> Hi David and David,
>>
>> I'm working on libvirt's support [1] for query-machines'
>> default-cpu-type, which is supposed to return the type of the default
>> CPU model that QEMU uses for each machine type. Rather than hard coding
>> the default in libvirt (which we currently do on x86), we ask QEMU for
>> the default CPU model and use it unless a user asks for a specific CPU
>> model explicitly.
>>
>> We use query-cpu-definitions for translating the default CPU type to the
>> actual CPU model we need to pass to -cpu by looking up the CPU model
>> with matching typename.
>>
>> However, all this seems to work only with TCG on both s390x and ppc64.
>> The issues I met with KVM on each architecture are described below.
>>
>> On ppc64 the default CPU type reported by query-machines is
>> power*-powerpc64-cpu, while IIUC QEMU would effectively use -cpu host by
>> default. In fact -cpu power8 is mostly just a fancy alias to -cpu host
>> on a Power8 machine. But QEMU even rewrites typename of the current host
>> CPU model to host-powerpc64-cpu. Which means on a Power8 host the power8
>> CPU model would have typename=host-powerpc64-cpu while the default CPU
>> type would still use power8*-powerpc64-cpu. Thus we may just fail to
>> find any CPU model corresponding to the default CPU model.
>>
>> And to make it even worse, the default CPU model changes with machine
>> type. E.g., pseries-3.1 uses power8_v2.0-powerpc64-cpu while pseries-4.2
>> uses power9_v2.0-powerpc64-cpu. However, starting QEMU with pseries-4.2
>> machine type and the reported default -cpu power9 fails on any
>> non-Power9 host. That said QEMU just lies about the default CPU model.
>>
>> So for all combinations of (pseries-3.1, pseries-4.2) machine types and
>> (Power8, Power9) hosts, one will get the following mixed results:
>>
>> - pseries-3.1 on Power8: no default CPU model will be detected, QEMU
>>    starts fine with it's own default
>> - pseries-4.2 on Power9: same as above
>> - pseries-3.1 on Power9: -cpu power8 (not sure if this works, though)
>> - pseries-4.2 on Power8: -cpu power9, QEMU doesn't start
>>
>>
>> This situation on s390x is not so complicated, but not really better.
>> The default CPU is said to be "qemu" for all machine types, which works
>> fine for TCG domains, but it doesn't work on KVM because QEMU complains
>> that some features requested in the CPU model are not available. In
>> other words the "qemu" CPU model is not runnable on KVM. This a bit
>> similar to what happens on x86_64, but QEMU just ignores missing
>> features and starts happily there.
> 
> The default model under KVM is "host", under TCG it's "qemu". We should not 
> use "qemu" under KVM, although it might work on some setups ...
> 
> Where/how is this default model detected?

I would prefer it libvirt would use the equivalent of mode=host-model by
default. Is this what this series is all about?