Re: [PATCH v1 17/55] KVM: PPC: Book3S HV P9: Implement PMU save/restore in C

Excerpts from Athira Rajeev's message of August 9, 2021 1:03 pm:

quoted

On 26-Jul-2021, at 9:19 AM, Nicholas Piggin [off-list ref] wrote:

quoted

+static void freeze_pmu(unsigned long mmcr0, unsigned long mmcra)
+{
+	if (!(mmcr0 & MMCR0_FC))
+		goto do_freeze;
+	if (mmcra & MMCRA_SAMPLE_ENABLE)
+		goto do_freeze;
+	if (cpu_has_feature(CPU_FTR_ARCH_31)) {
+		if (!(mmcr0 & MMCR0_PMCCEXT))
+			goto do_freeze;
+		if (!(mmcra & MMCRA_BHRB_DISABLE))
+			goto do_freeze;
+	}
+	return;
+
+do_freeze:
+	mmcr0 = MMCR0_FC;
+	mmcra = 0;
+	if (cpu_has_feature(CPU_FTR_ARCH_31)) {
+		mmcr0 |= MMCR0_PMCCEXT;
+		mmcra = MMCRA_BHRB_DISABLE;
+	}
+
+	mtspr(SPRN_MMCR0, mmcr0);
+	mtspr(SPRN_MMCRA, mmcra);
+	isync();
+}
+

Hi Nick,

After feezing pmu, do we need to clear “pmcregs_in_use” as well?

Not until we save the values out of the registers. pmcregs_in_use = 0 
means our hypervisor is free to clear our PMU register contents.

Also can’t we unconditionally do the MMCR0/MMCRA/ freeze settings in here ? do we need the if conditions for FC/PMCCEXT/BHRB ?

I think it's possible, but pretty minimal advantage. I would prefer to 
set them the way perf does for now. If we can move this code into perf/
it should become easier for you to tweak things.

Thanks,
Nick