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

[Salil Mehta]

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.

static void fdt_add_pmu_nodes(const VirtMachineState *vms)
{
    CPUState *cpu;
    ARMCPU *armcpu;
    uint32_t irqflags = GIC_FDT_IRQ_FLAGS_LEVEL_HI;

    CPU_FOREACH(cpu) {
        armcpu = ARM_CPU(cpu);
        if (!arm_feature(&armcpu->env, ARM_FEATURE_PMU)) {
            return; ------> Here, loop exits & function returns
        }
        if (kvm_enabled()) {
            if (kvm_irqchip_in_kernel()) {
                kvm_arm_pmu_set_irq(cpu, PPI(VIRTUAL_PMU_IRQ));
            }
            kvm_arm_pmu_init(cpu);
        }
    }

    if (vms->gic_version == VIRT_GIC_VERSION_2) {
        irqflags = deposit32(irqflags, GIC_FDT_IRQ_PPI_CPU_START,
                             GIC_FDT_IRQ_PPI_CPU_WIDTH,
                             (1 << vms->smp_cpus) - 1);
    }

    armcpu = ARM_CPU(qemu_get_cpu(0));
    qemu_fdt_add_subnode(vms->fdt, "/pmu");
    if (arm_feature(&armcpu->env, ARM_FEATURE_V8)) {
        const char compat[] = "arm,armv8-pmuv3";
        qemu_fdt_setprop(vms->fdt, "/pmu", "compatible",
                         compat, sizeof(compat));
        qemu_fdt_setprop_cells(vms->fdt, "/pmu", "interrupts",
                               GIC_FDT_IRQ_TYPE_PPI, VIRTUAL_PMU_IRQ, irqflags);
    }
}

Questions:
Q1. Not sure what is the logic of the premature exit and not
    continuing with further checks and initialization of other
    VCPU PMUs?
Q2. Does it even makes sense to have PMUs initialized for some
    vcpus and not for others unless we have heterogeneous system? 
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?
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?


Best regards
Salil