Re: [Qemu-devel] [PATCH for-2.9 15/17] target-i386: Define static "base" CPU model

[David Hildenbrand <david@redhat.com>]

Am 06.12.2016 um 00:57 schrieb Eduardo Habkost:
> On Mon, Dec 05, 2016 at 07:18:47PM +0100, David Hildenbrand wrote:
>> Am 02.12.2016 um 22:18 schrieb Eduardo Habkost:
>>> The query-cpu-model-expand QMP command needs at least one static
>>> model, to allow the "static" expansion mode to be implemented.
>>> Instead of defining static versions of every CPU model, define a
>>> "base" CPU model that has absolutely no feature flag enabled.
>>>
>>
>> Introducing separate ones makes feature lists presented to the user much
>> shorter (and therefore easier to maintain). But I don't know how libvirt
>> wants to deal with models on x86 in the future.
>
> I understand that having a larger set of static models would make
> expansions shorter. But I worry that by defining a complete set
> of static models on x86 would require extra maintenance work on
> the QEMU side with no visible benefit for libvirt.

As static models will never change (theory) the maintenance work should 
be pretty much down to zero. But the initial implementation and comming 
up with the models requires work (my experience ;) ).

I am not against the "base" model (actually it is really pretty nice to 
have). Using only that somehow smells like the "user" cpu model 
discussion. Which might be ok for x86.

>
> I would like to hear from libvirt developers what they think. I
> still don't know what they plan to use the type=static expansion
> results for.
>
>>
>> How long is the static expansion on a recent intel CPU?
>
> CPU model "Broadwell" returns 165 entries on return.model.props:
>
> (QEMU) query-cpu-model-expansion type=static model={"name":"Broadwell"}

> {"return": {"migration-safe": true, "model": {"name": "base", "props": {"pfthreshold": false, "pku": false, "rtm": true, "tsc-deadline": true, "xstore-en": false, "tsc-scale": false, "abm": true, "ia64": false, "kvm-mmu": false, "xsaveopt": true, "tce": false, "smep": true, "fpu": true, "xcrypt": false, "clflush": true, "flushbyasid": false, "kvm-steal-time": false, "lm": true, "tsc": true, "adx": true, "fxsr": true, "tm": false, "xgetbv1": false, "xstore": false, "vme": false, "vendor": "GenuineIntel", "arat": true, "de": true, "aes": true, "pse": true, "ds-cpl": false, "tbm": false, "sse": true, "phe-en": false, "f16c": true, "ds": false, "mpx": false, "tsc-adjust": false, "avx512f": false, "avx2": true, "pbe": false, "cx16": true, "avx512pf": false, "movbe": true, "perfctr-nb": false, "ospke": false, "avx512ifma": false, "stepping": 2, "sep": true, "sse4a": false, "avx512dq": false, "avx512-4vnniw": false, "xsave": true, "pmm": false, "hle": true, "est": false, "xop": false, "smx": false, "monitor": false, "avx512er": false, "apic": true, "sse4.1": true, "sse4.2": true, "pause-filter": false, "lahf-lm": true, "kvm-nopiodelay": false, "acpi": false, "mmx": true, "osxsave": false, "pcommit": false, "mtrr": true, "clwb": false, "dca": false, "pdcm": false, "xcrypt-en": false, "3dnow": false, "invtsc": false, "tm2": false, "hypervisor": true, "kvmclock-stable-bit": false, "fxsr-opt": false, "pcid": true, "lbrv": false, "avx512-4fmaps": false, "svm-lock": false, "popcnt": true, "nrip-save": false, "avx512vl": false, "x2apic": true, "kvmclock": false, "smap": true, "family": 6, "min-level": 13, "dtes64": false, "ace2": false, "fma4": false, "xtpr": false, "avx512bw": false, "nx": true, "lwp": false, "msr": true, "ace2-en": false, "decodeassists": false, "perfctr-core": false, "pge": true, "pn": false, "fma": true, "nodeid-msr": false, "cx8": true, "mce": true, "avx512cd": false, "cr8legacy": false, "mca": true, "pni": true, "rdseed": true, "osvw": false, "fsgsbase": true, "model-id": "Intel Core Processor (Broadwell)", "cmp-legacy": false, "kvm-pv-unhalt": false, "rdtscp": true, "mmxext": false, "cid": false, "vmx": false, "ssse3": true, "extapic": false, "pse36": true, "min-xlevel": 2147483656, "ibs": false, "avx": true, "syscall": true, "umip": false, "invpcid": true, "bmi1": true, "bmi2": true, "vmcb-clean": false, "erms": true, "cmov": true, "misalignsse": false, "clflushopt": false, "pat": true, "3dnowprefetch": true, "rdpid": false, "pae": true, "wdt": false, "skinit": false, "pmm-en": false, "phe": false, "3dnowext": false, "lmce": false, "ht": false, "pdpe1gb": false, "kvm-pv-eoi": false, "npt": false, "xsavec": false, "pclmulqdq": true, "svm": false, "sse2": true, "ss": false, "topoext": false, "rdrand": true, "avx512vbmi": false, "kvm-asyncpf": false, "xsaves": false, "model": 61}}, "static": true}}
>
>

Wow, yes that was the reason for me to introduce abstractions on s390x. 
But here the plan was to use the epansion directly when indication the
"host" model to the user. Having something like "Broadwell-base"+/- a 
handful of features is just easier to handle than "base" with 165 
feature flags. But as we don't know what libvirt plans are (they could 
use that interface on x86 to do feature detection only and convert to 
models themselves), I also have no idea what would be best in the 
context of x86 cpu models.

-- 

David