Hello,
Any comments on this would be really helpful & much appreciated.
Thanks in anticipation!
Best regards
Salil
> -----Original Message-----
> From: Salil Mehta
> Sent: Monday, June 1, 2020 4:00 PM
> To: qemu-devel@nongnu.org; qemu-...@nongnu.org
> Cc: Peter Maydell <peter.mayd...@linaro.org>; m...@redhat.com; Igor Mammedov
> <imamm...@redhat.com>
> Subject: [Question] Regarding PMU initialization within the QEMU for ARM VCPUs
>
> 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
>
