linux-kernel - Re: [RFC 1/3] x86: KVM: stats: Add a stat to report status of APICv inhibition

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Date: Tue, 9 Apr 2024 21:54:37 +0700
From: "Suthikulpanit, Suravee" <suravee.suthikulpanit@....com>
To: Alejandro Jimenez <alejandro.j.jimenez@...cle.com>, kvm@...r.kernel.org
Cc: seanjc@...gle.com, pbonzini@...hat.com, linux-kernel@...r.kernel.org,
 joao.m.martins@...cle.com, boris.ostrovsky@...cle.com,
 mark.kanda@...cle.com, mlevitsk@...hat.com
Subject: Re: [RFC 1/3] x86: KVM: stats: Add a stat to report status of APICv
 inhibition

Hi

On 2/15/2024 11:01 PM, Alejandro Jimenez wrote:
> The inhibition status of APICv can currently be checked using the
> 'kvm_apicv_inhibit_changed' tracepoint, but this is not accessible if
> tracefs is not available (e.g. kernel lockdown, non-root user). Export
> inhibition status as a binary stat that can be monitored from userspace
> without elevated privileges.
> 
> Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@...cle.com>
> ---
>   arch/x86/include/asm/kvm_host.h |  1 +
>   arch/x86/kvm/x86.c              | 10 +++++++++-
>   2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index ad5319a503f0..9b960a523715 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1524,6 +1524,7 @@ struct kvm_vm_stat {
>   	u64 nx_lpage_splits;
>   	u64 max_mmu_page_hash_collisions;
>   	u64 max_mmu_rmap_size;
> +	u64 apicv_inhibited;
>   };
>   
>   struct kvm_vcpu_stat {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b66c45e7f6f8..f7f598f066e7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -255,7 +255,8 @@ const struct _kvm_stats_desc kvm_vm_stats_desc[] = {
>   	STATS_DESC_ICOUNTER(VM, pages_1g),
>   	STATS_DESC_ICOUNTER(VM, nx_lpage_splits),
>   	STATS_DESC_PCOUNTER(VM, max_mmu_rmap_size),
> -	STATS_DESC_PCOUNTER(VM, max_mmu_page_hash_collisions)
> +	STATS_DESC_PCOUNTER(VM, max_mmu_page_hash_collisions),
> +	STATS_DESC_IBOOLEAN(VM, apicv_inhibited)

One use for this stats would be to help:
1. Determine if vm and/or vcpu is inhibited.
2. Determine the inhibit reason.

Therefore can we use STATS_DESC_ICOUNTER() instead of 
STATS_DESC_IBOOLEAN(), and show the inhibit flag from

vm_reason   = struct kvm->arch.apicv_inhibit_reasons
vcpu_reason = static_call(kvm_x86_vcpu_get_apicv_inhibit_reasons)(vcpu)

See kvm_vcpu_apicv_activated().

Thanks,
Suravee

>   };
>   
>   const struct kvm_stats_header kvm_vm_stats_header = {
> @@ -10588,6 +10589,13 @@ void __kvm_set_or_clear_apicv_inhibit(struct kvm *kvm,
>   		 */
>   		kvm_make_all_cpus_request(kvm, KVM_REQ_APICV_UPDATE);
>   		kvm->arch.apicv_inhibit_reasons = new;
> +
> +		/*
> +		 * Update inhibition statistic only when toggling APICv
> +		 * activation status.
> +		 */
> +		kvm->stat.apicv_inhibited = !!new;
> +
>   		if (new) {
>   			unsigned long gfn = gpa_to_gfn(APIC_DEFAULT_PHYS_BASE);
>   			int idx = srcu_read_lock(&kvm->srcu);