Subject: Re: [PATCH v2 1/3] KVM: arm64: Add a stage2 page fault counter for kvm_stat

From: Marc Zyngier <[email-masked]>
Date: Tue, 20 Apr 2021 14:31:05 +0100

On Tue, 20 Apr 2021 14:08:23 +0100,
Yoan Picchi <[email-masked]> wrote:
> 
> This counter is meant to detect stage 2 page fault exits. This is meant to
> measure how much those page fault influence the performance (by comparing
> this number of exits to some other exit causes).
> For now this counter is generic, but some more granularity is planned in
> the next commits so that one know how much of the page fault are for memory
> allocation, or for mmio for instance. The idea being that one using this
> counter can get a better idea of what is trigerring those exits to try to
> fix it.
> 
> Signed-off-by: Yoan Picchi <[email-masked]>
> ---
>  arch/arm64/include/asm/kvm_host.h | 1 +
>  arch/arm64/kvm/guest.c            | 1 +
>  arch/arm64/kvm/mmu.c              | 2 ++
>  3 files changed, 4 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 3d10e6527..02891ce94 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -561,6 +561,7 @@ struct kvm_vcpu_stat {
>  	u64 wfi_exit_stat;
>  	u64 mmio_exit_user;
>  	u64 mmio_exit_kernel;
> +	u64 stage2_abort_exit;
>  	u64 exits;
>  };
>  
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 9bbd30e62..82a4b6275 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -38,6 +38,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
>  	VCPU_STAT("wfi_exit_stat", wfi_exit_stat),
>  	VCPU_STAT("mmio_exit_user", mmio_exit_user),
>  	VCPU_STAT("mmio_exit_kernel", mmio_exit_kernel),
> +	VCPU_STAT("stage2_abort_exit", stage2_abort_exit),
>  	VCPU_STAT("exits", exits),
>  	VCPU_STAT("halt_poll_success_ns", halt_poll_success_ns),
>  	VCPU_STAT("halt_poll_fail_ns", halt_poll_fail_ns),
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 77cb2d28f..c3527ccf6 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -966,6 +966,7 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
>  	fault_ipa = kvm_vcpu_get_fault_ipa(vcpu);
>  	is_iabt = kvm_vcpu_trap_is_iabt(vcpu);
>  
> +

Please proofread your patches, and don't add spurious blank lines.

>  	/* Synchronous External Abort? */
>  	if (kvm_vcpu_abt_issea(vcpu)) {
>  		/*
> @@ -980,6 +981,7 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
>  
>  	trace_kvm_guest_fault(*vcpu_pc(vcpu), kvm_vcpu_get_esr(vcpu),
>  			      kvm_vcpu_get_hfar(vcpu), fault_ipa);
> +	vcpu->stat.stage2_abort_exit++;

What is the benefit of this counter over, say, the tracepoint that is
already in place? This tracepoint allows you to count these exits, and
to actually find *why* you are exiting.

If the purpose of this patch is to identify exit reasons, I'd say that
the tracepoint is vastly superior in that respect.

	M.

-- 
Without deviation from the norm, progress is not possible.

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