Re: [Qemu-devel] [RFC PATCH] target/s390x/cpu_models: Set some additional feature bits for the "qemu" CPU

[Thomas Huth <thuth@redhat.com>]

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