Re: arm_cpu_post_init (Was: Re: arm: "max" CPU class hierarchy changes possible?)

[Claudio Fontana <cfontana@suse.de>]

On 3/19/21 9:23 AM, Claudio Fontana wrote:
> Hi Markus,
> 
> could you help me untangle the arm_cpu_post_init question?

Nevermind, I think I figured it out. The arm_cpu_post_init are indeed called only for the "leaf" class,
via the "instance_init" functions.

I think I can use it to do things reliably "post init" for all classes in there.

Thanks,

Claudio

> 
> I am trying to cleanup a bit the initialization path for ARM,
> and it seems that arm_cpu_post_init is called numerous times for AArch64 in particular,
> 
> while for "tcg cpus", 32bit it is called only once.
> 
> Any reason for the multiple calls in the hierarchy?
> Was the intention to actually call this just once from the final leaf classes?
> 
> The ability to execute code after the initialization would come in handy in an ARM CPU class refactoring I am doing,
> but I stopped short of adding anything to arm_cpu_post_init since I noticed the inconsistencies.
> 
> Thanks,
> 
> Claudio
> 
> 
> On 3/18/21 12:06 PM, Claudio Fontana wrote:
>> On 3/11/21 8:10 PM, Andrew Jones wrote:
>>> On Thu, Mar 11, 2021 at 06:33:15PM +0000, Peter Maydell wrote:
>>>> On Thu, 11 Mar 2021 at 17:16, Claudio Fontana <cfontana@suse.de> wrote:
>>>>> Maybe Peter you could clarify similarly what the intended meaning of "max" is on ARM?
>>>>
>>>> "max" is "best we can do, whatever that is". (On KVM this is "same as
>>>> the host".)
>>>> "host" is "whatever the host is (KVM only)".
>>>>
>>>>> KVM: (aarch64-only): aarch64_max_initfn():
>>>>>
>>>>> The following comment in the code seems wrong to me:
>>>>>
>>>>> /* -cpu max: if KVM is enabled, like -cpu host (best possible with this host); */
>>>>>
>>>>> This is not exactly true:
>>>>>
>>>>> "-cpu max" calls kvm_arm_set_cpu_features_from_host(), (which checks "dtb_compatible", and if not set gets the features from the host, if set ...?)
>>>>> After that, calls aarch64_add_sve_properties() and then adds also "svw-max-vq". This code is common with TCG.
>>
>>
>> As part of this research I noticed that arm_cpu_post_init() is quite confusing, seems really inconsistent to me.
>>
>> Apparently the intention was to call it from the leaf classes:
>>
>> commit 51e5ef459eca045d7e8afe880ee60190f0b75b26
>> Author: Marc-André Lureau <marcandre.lureau@redhat.com>
>> Date:   Tue Nov 27 12:55:59 2018 +0400
>>
>>     arm: replace instance_post_init()
>>     
>>     Replace arm_cpu_post_init() instance callback by calling it from leaf
>>     classes, to avoid potential ordering issue with other post_init callbacks.
>>     
>>     Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
>>     Suggested-by: Igor Mammedov <imammedo@redhat.com>
>>     Reviewed-by: Igor Mammedov <imammedo@redhat.com>
>>     Acked-by: Eduardo Habkost <ehabkost@redhat.com>
>>
>>
>> but then we end up calling it multiple times in the class hierarch, which is a recipe for bugs, and makes it difficult to understand what arm_cpu_post_init()
>> even means, what calling this function is supposed to do.
>>
>> For a "max" or "host" cpu on AArch64, this function is called:
>>
>> for the ARM CPU base class, TYPE_ARM_CPU, in
>>
>> cpu.c::arm_cpu_instance_init,
>>
>> then later again for the TYPE_AARCH64_CPU class, child of TYPE_ARM_CPU, in
>>
>> cpu64.c::aarch64_cpu_instance_init,
>>
>> then later again for the TYPE_ARM_HOST_CPU class, child of TYPE_AARCH64_CPU, in
>>
>> cpu.c::arm_host_initfn.
>>
>> Same for "max".
>>
>> When looking at 32bit CPUs instead, only the ARM CPU base class ends up calling arm_cpu_post_init.
>> "Leaf" classes do not do it (see cpu_tcg.c).
>>
>> What is then arm_cpu_post_init even supposed to mean?
>>
>> Thanks,
>>
>> Claudio
>>
>>
>>>>>
>>>>> In the case of cpu host instead,
>>>>>
>>>>> "-cpu host" calls kvm_arm_set_cpu_features_from_host(), same as max, then calls aarch64_add_sve_properties() but does NOT add "svw-max-vq".
>>>>>
>>>>> Is this a bug?
>>>
>>> It was left out intentionally. More below.
>>>
>>>>
>>>> Maybe; that's a question for Richard or Drew...
>>>>
>>>>> Are "max" and "host" for KVM supposed to be the same like with x86?
>>>
>>> Yes, but my understanding of "max" == "host" for KVM is that that only
>>> applies to the perspective of the guest. What CPU and what CPU features
>>> the guest can see should be exactly the same with either "max" or "host",
>>> depending on the enabling/disabling of any optional CPU properties.
>>>
>>> The question here seems to be that, if one has a CPU property, does that
>>> imply the other should have the same? Which would effectively allow the
>>> two to be aliases (when KVM is enabled). I don't know, does x86 ensure
>>> 100% property compatibility?
>>>
>>> I opted not to support sve-max-vq for "host" because I consider it a
>>> legacy CPU property, one I didn't want to propagate. Indeed it may
>>> make more sense to depreciate sve-max-vq than to "fix" this issue
>>> by adding it to "host". Note, we can already create equivalent SVE
>>> CPUs. The following are the same from the perspective of the guest
>>>
>>>  -accel kvm -cpu host,sve512=on
>>>  -accel kvm -cpu max,sve512=on
>>>
>>> And, for TCG, these are the same from the perspective of the guest
>>>  
>>>  -accel tcg -cpu max,sve512=on
>>>  -accel tcg -cpu max,sve-max-vq=4
>>>
>>> So we already don't need sve-max-vq.
>>>
>>> Thanks,
>>> drew
>>>
>>
>>
> 
>