Re: [Qemu-devel] [PATCH 00/10] s390x: new guest features

[David Hildenbrand]

On 24.04.19 12:26, Christian Borntraeger wrote:
> 
> 
> On 24.04.19 11:56, David Hildenbrand wrote:
>> On 24.04.19 11:41, Daniel P. Berrangé wrote:
>>> On Wed, Apr 24, 2019 at 11:35:40AM +0200, David Hildenbrand wrote:
>>>> On 24.04.19 11:30, Daniel P. Berrangé wrote:
>>>>> On Wed, Apr 24, 2019 at 11:03:03AM +0200, David Hildenbrand wrote:
>>>>>> On 24.04.19 10:40, Christian Borntraeger wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 23.04.19 14:11, David Hildenbrand wrote:
>>>>>>>> On 18.04.19 13:31, Christian Borntraeger wrote:
>>>>>>>>> Adding generation 15.
>>>>>>>>>
>>>>>>>>> Some interesting aspects:
>>>>>>>>> - conditional SSKE and bpb are deprecated. This patch set addresses 
>>>>>>>>> that
>>>>>>>>>   for csske.
>>>>>>>>> - no name yet for gen15, I suggest to use the assigned numbers and
>>>>>>>>>   provide an alias later on. (I have split out this into a separate
>>>>>>>>>   patch)
>>>>>>>>>
>>>>>>>>> Christian Borntraeger (10):
>>>>>>>>>   linux header sync
>>>>>>>>>   s390x/cpumodel: remove CSSKE from base model
>>>>>>>>>   s390x/cpumodel: Miscellaneous-Instruction-Extensions Facility 3
>>>>>>>>>   s390x/cpumodel: msa9 facility
>>>>>>>>>   s390x/cpumodel: vector enhancements
>>>>>>>>>   s390x/cpumodel: enhanced sort facility
>>>>>>>>>   s390x/cpumodel: deflate
>>>>>>>>>   s390x/cpumodel: add gen15 defintions
>>>>>>>>>   s390x/cpumodel: wire up 8561 and 8562 as gen15 machines
>>>>>>>>>   s390x/cpumodel: do not claim csske for expanded models in qmp
>>>>>>>>>
>>>>>>>>>  hw/s390x/s390-virtio-ccw.c      |  6 +++
>>>>>>>>>  linux-headers/asm-s390/kvm.h    |  5 +-
>>>>>>>>>  target/s390x/cpu_features.c     | 54 +++++++++++++++++++
>>>>>>>>>  target/s390x/cpu_features.h     |  3 ++
>>>>>>>>>  target/s390x/cpu_features_def.h | 49 +++++++++++++++++
>>>>>>>>>  target/s390x/cpu_models.c       | 35 ++++++++++++
>>>>>>>>>  target/s390x/cpu_models.h       |  1 +
>>>>>>>>>  target/s390x/gen-features.c     | 94 
>>>>>>>>> ++++++++++++++++++++++++++++++++-
>>>>>>>>>  target/s390x/kvm.c              | 18 +++++++
>>>>>>>>>  9 files changed, 263 insertions(+), 2 deletions(-)
>>>>>>>>>
>>>>>>>>
>>>>>>>> I guess to handle deprecation of CSSKE:
>>>>>>>>
>>>>>>>> 1. Remove it from the base + default model of the gen15, keep it in the
>>>>>>>> max model. This is completely done in target/s390x/gen-features.c.
>>>>>>>> Existing base models are not modified.
>>>>>>>>
>>>>>>>> 2. Add CSSKE to "ignored_base_feat", so fallback of gen15 to e.g. z14
>>>>>>>> will work. We can backport this to distros/stable.
>>>>>>>
>>>>>>> Yes, I have already implemented that, still doing some testing and 
>>>>>>> polishinh.
>>>>>>>>
>>>>>>>>
>>>>>>>> CPU model expansion:
>>>>>>>>
>>>>>>>> cpu_info_from_model() should already properly be based on the base
>>>>>>>> features. "gen15" vs. "gen15,csske=on" should be automatically 
>>>>>>>> generated
>>>>>>>> when expanding.
>>>>>>>>
>>>>>>>> CPU model baseline:
>>>>>>>>
>>>>>>>> s390_find_cpu_def() should make sure that CSSKE is basically ignored
>>>>>>>> when determining maximum model, however it will properly be indicated 
>>>>>>>> if
>>>>>>>> both models had the feature.
>>>>>>>>
>>>>>>>> CPU model comparison:
>>>>>>>>
>>>>>>>> Should work as expected. Availability of CSSKE will be considered when
>>>>>>>> calculating the result.
>>>>>>>>
>>>>>>>> gen14,csske=on and gen15,csske=off will result in
>>>>>>>> CPU_MODEL_COMPARE_RESULT_INCOMPATIBLE.
>>>>>>>>
>>>>>>>> gen14,csske=off and gen15,csske=off should result in
>>>>>>>> CPU_MODEL_COMPARE_RESULT_SUBSET
>>>>>>>>
>>>>>>>> gen14,csske=on and gen15,csske=on should result in
>>>>>>>> CPU_MODEL_COMPARE_RESULT_SUBSET
>>>>>>>>
>>>>>>>> Forward migration:
>>>>>>>>
>>>>>>>> Now, the only issue is when csske is actually turned of in future
>>>>>>>> machines. We would e.g. have
>>>>>>>>
>>>>>>>> gen15,csske=on and gen16,csske=off
>>>>>>>>
>>>>>>>> While baselining will work correctly (gen15,csske=off), forward
>>>>>>>> migration is broken (comparison will properly indicate
>>>>>>>> CPU_MODEL_COMPARE_RESULT_INCOMPATIBLE), which is expected when ripping
>>>>>>>> out features. Same applies to BPB.
>>>>>>>>
>>>>>>>>
>>>>>>>> Your patch "[PATCH 10/10] s390x/cpumodel: do not claim csske for
>>>>>>>> expanded models in qmp" tried to address this, however I am not really
>>>>>>>> happy with this approach. We should not play such tricks when expanding
>>>>>>>> the host model. "-cpu host" and "-cpu $expanded_host" would be
>>>>>>>> different,
>>>>>>>
>>>>>>> We discussed this some time ago and I think we agreed that for host 
>>>>>>> passthrough
>>>>>>> it is ok to be different that host-model (e.g. passing through the 
>>>>>>> cpuid, passing
>>>>>>> through all non-hypervisor managed features etc).
>>>>>>
>>>>>> I remember the plan was to use the "max" model to do such stuff. E.g.
>>>>>> -cpu max / no -cpu
>>>>>>
>>>>>> Versus
>>>>>> -cpu host
>>>>>>
>>>>>> We can have features in "host" we don't have in "max". But "-cpu host"
>>>>>> and it's expansion should look 100% the same.
>>>>>
>>>>> I don't think that's the intended semantics of "max" vs "host".
>>>>>
>>>>> The "max" CPU model is supposed to enable all features that are possible 
>>>>> to
>>>>> enable.
>>>>>
>>>>> For KVM, thus "max" should be identical to "host".
>>>>
>>>> There once was a mode used by x86-64 to simply pipe through cpuid
>>>> features that were not even supported. (I remember something like
>>>> passthorugh=true), do you remember if something like that still exists?
>>>
>>> I don't recall such a feature existing even in the past !
>>
>> Maybe my mind is tricking me, or maybe that has long been removed :)
>>
>>>
>>>>> For TCG, "max" should be everything that QEMU currently knows how to 
>>>>> emulate.
>>>>
>>>> Yes, and on s390x. "max" contains more features than "qemu".
>>>>
>>>>>
>>>>> Essentially think of "max" as a better name for "host", since "host" as
>>>>> a name concept didn't make sense for TCG.
>>>>
>>>> I agree. The main question is, is it acceptable that
>>>
>>> Hmm, maybe I misinterpreted when you wrote
>>>
>>>       We can have features in "host" we don't have in "max"
>>>
>>> I read that as meaning that "-cpu host" and "-cpu max" would be
>>> different.
>>
>> No you didn't misinterpret it, I agreed after you spelled it out :)
>>
>>>
>>>> "-cpu host" and "-cpu $expanded_host" produce different results, after
>>>> expanding "host" via query-cpu-model-expansion?
>>>
>>> That has always been the case on x86-64, since it is not possible to set
>>> the level, xlevel, vendor, family, model & stepping properties via -cpu,
>>> only the feature flags.
>>
>> Fair enough, but the question is if we should mess with feature flags we
>> can indicate on that level.
>>
>> It would mean that on a specific host e.g.
>>
>> "-cpu gen15,csske=on" and "-cpu gen15,csske=off"
>>
>> Would work. However, "host" model expansion would give us
>>
>> "-cpu gen15,csske=off"
>>
>> So trying to e.g. do a query-cpu-model-comparison or
>> query-cpu-model-baseline against "host" and against the expanded host
>> model will produce different results.
>>
>> Libvirt could detect "-cpu gen14,csske=on" as not runnable on the host,
>> because comparing "-cpu gen14,csske=on" vs. "-cpu gen15,csske=off" would
>> be "incompatible". But running "-cpu gen14,csske=on" on the host would
>> work perfectly fine.
> 
> I would like to avoid special knowledge in libvirt (since we moved to have
> everything in qemu). 
> 
> A more complex idea would be to extend the qmp query with a list of deprecated
> features and libvirt could then disable that for expansion but allow it for
> baselining.

That would fit in nicely into query-cpu-model-expansion. Nice idea.

-- 

Thanks,

David / dhildenb