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

[Salil Mehta]

Hi Eric,

> From: Qemu-arm [mailto:qemu-arm-bounces+salil.mehta=huawei.com@nongnu.org]
> On Behalf Of Auger Eric
> Sent: Wednesday, June 3, 2020 10:59 AM
> To: Andrew Jones <drjones@redhat.com>; Salil Mehta <salil.mehta@huawei.com>
> Cc: Peter Maydell <peter.maydell@linaro.org>; Igor Mammedov
> <imammedo@redhat.com>; qemu-arm@nongnu.org; qemu-devel@nongnu.org;
> mst@redhat.com
> Subject: Re: [Question] Regarding PMU initialization within the QEMU for ARM
> VCPUs
> 
> 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


Indeed. It is being set on per-vcpu level but actually all of
the vcpus will either have ON or OFF PMU feature - at least for
now. Not sure if we ever want to change this in future?

Thanks
Salil.