Re: [Question] Regarding PMU initialization within the QEMU for ARM VCPUs

[Auger Eric]

Hi Drew,

On 6/3/20 11:37 AM, Andrew Jones wrote:
> On Mon, Jun 01, 2020 at 03:04:33PM +0000, Salil Mehta wrote:
>> Hello,
>> I could see below within function fdt_add_pmu_nodes() part of
>> hw/arm/virt.c during virt machine initialization time:
>>
>> Observation:
>> In below function, support of PMU feature is being checked for
>> each vcpu and if the PMU is found part of the features then PMU
>> is initialized with in the host/KVM. But if there is even one
>> vcpu which is found to not support the PMU then loop is exited
>> and PMU is not initialized for the rest of the vcpus as well.
>>
>> Questions:
>> Q1. Not sure what is the logic of the premature exit and not
>>     continuing with further checks and initialization of other
>>     VCPU PMUs?
> 
> KVM requires all VCPUs to have a PMU if one does.

I fail to find where this is enforced? Do you know the place?

 If the ARM ARM
> says it's possible to have PMUs for only some CPUs, then, for TCG,
> the restriction could be relaxed. I expect it will take more than
> just removing the check for things to work though.>
>> Q2. Does it even makes sense to have PMUs initialized for some
>>     vcpus and not for others unless we have heterogeneous system? 
> 
> I don't know, but it doesn't sound like a configuration I'd like
> to see.
> 
>> Q3. Also, there is a per virt machine knob of vcc->no_pmu.
>>     This is something which user could specify at the init time
>>     and perhaps only once but we don't use it for ARM. Perhaps
>>     should have been used even before entering this function
>>     to enable or disable the support as per user config?
> 
> It's purpose is to keep users from doing 'pmu=on' on 2.6 machine
> types. On 2.7 and later machine types if you don't want a PMU
> you should use 'pmu=off'.

extra note:
the cpu pmu property sets the feature at vcpu level. This is what is
retrieved when (!arm_feature(&armcpu->env, ARM_FEATURE_PMU)) gets called.

See the cpu option setter: arm_set_pmu in target/arm/cpu.c

> 
>> Q4. This function  fdt_* looks to be wrongly named. The info
>>     being initialized here shall be used even when ACPI is
>>     being used. Initialization part and FDT info looked
>>     mixed up here if I am right?
> 
> Agreed. The function has the wrong name. mach-virt has many functions that
> mix the initialization and fdt building together, but those functions are
> named something like create_foo(). Patches welcome.
agreed

Thanks

Eric
> 
> Thanks,
> drew
> 
>