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

[Andrew Jones]

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. 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'.

> 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.

Thanks,
drew