[PATCH] KVM: arm64: Replace KVM_ARM_PMU with HW_PERF_EVENTS

[Alexandru Elisei]

Hi Marc,

On 1/4/21 5:27 PM, Marc Zyngier wrote:
> KVM_ARM_PMU only existed for the benefit of 32bit ARM hosts,
> and makes no sense now that we are 64bit only. Get rid of it.
>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kvm/Kconfig  | 8 --------
>  arch/arm64/kvm/Makefile | 2 +-
>  include/kvm/arm_pmu.h   | 2 +-
>  3 files changed, 2 insertions(+), 10 deletions(-)
>
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 043756db8f6e..3964acf5451e 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -49,14 +49,6 @@ if KVM
>  
>  source "virt/kvm/Kconfig"
>  
> -config KVM_ARM_PMU
> -	bool "Virtual Performance Monitoring Unit (PMU) support"
> -	depends on HW_PERF_EVENTS
> -	default y
> -	help
> -	  Adds support for a virtual Performance Monitoring Unit (PMU) in
> -	  virtual machines.
> -
>  endif # KVM
>  
>  endif # VIRTUALIZATION
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index 60fd181df624..13b017284bf9 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -24,4 +24,4 @@ kvm-y := $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o $(KVM)/eventfd.o \
>  	 vgic/vgic-mmio-v3.o vgic/vgic-kvm-device.o \
>  	 vgic/vgic-its.o vgic/vgic-debug.o
>  
> -kvm-$(CONFIG_KVM_ARM_PMU)  += pmu-emul.o
> +kvm-$(CONFIG_HW_PERF_EVENTS)  += pmu-emul.o
> diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> index fc85f50fa0e9..8dcb3e1477bc 100644
> --- a/include/kvm/arm_pmu.h
> +++ b/include/kvm/arm_pmu.h
> @@ -13,7 +13,7 @@
>  #define ARMV8_PMU_CYCLE_IDX		(ARMV8_PMU_MAX_COUNTERS - 1)
>  #define ARMV8_PMU_MAX_COUNTER_PAIRS	((ARMV8_PMU_MAX_COUNTERS + 1) >> 1)
>  
> -#ifdef CONFIG_KVM_ARM_PMU
> +#ifdef CONFIG_HW_PERF_EVENTS
>  
>  struct kvm_pmc {
>  	u8 idx;	/* index into the pmu->pmc array */

I grep'ed for KVM_ARM_PMU in the Linux sources, this patch takes care of all its
occurrences.

A few things popped into my mind when I saw the patch.

1. Replacing KVM_ARM_PMU with CONFIG_HW_PERF_EVENTS means it's not possible
anymore for the host to have perf support while KVM does not support emulating a
PMU. In this scenario, functions which would have been empty functions if
KVM_ARM_PMU was still around (I only found kvm_pmu_flush_hwstate() and
kvm_pmu_sync_hwstate() on the KVM_RUN path) will now be called and return early
after kvm_vcpu_has_pmu() returns 0. The overhead looks negligible to me, and I
don't think this configuration was common (especially since the default was y).

2. I did a grep for the files that include arm_pmu.h, and all the files were in
arch/arm64. I suppose arm_pmu.h exists in include/kvm instead of
arch/arm64/include/asm because it was shared with KVM/arm when it was still
around, right? Or is there another reason for that?

[1] https://www.spinics.net/lists/kvm-arm/msg44184.html

Thanks,
Alex