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

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Date: Tue, 16 Apr 2024 11:19:38 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Alejandro Jimenez <alejandro.j.jimenez@...cle.com>
Cc: kvm@...r.kernel.org, pbonzini@...hat.com, linux-kernel@...r.kernel.org, 
	joao.m.martins@...cle.com, boris.ostrovsky@...cle.com, mark.kanda@...cle.com, 
	suravee.suthikulpanit@....com, mlevitsk@...hat.com
Subject: Re: [RFC 1/3] x86: KVM: stats: Add a stat to report status of APICv inhibition

On Thu, Feb 15, 2024, 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;

Tracking the negative is odd, i.e. if we add a stat, KVM should probably track
if APICv is fully enabled, not if it's inhibited.

This also should be a boolean, not a u64.  Precisely enumerating _why_ APICv is
inhibited is firmly in debug territory, i.e. not in scope for "official" stats.

Oh, and this should be a per-vCPU stat, not a VM-wide stat.

As for whether or not we should add a stat for this, I'm leaning towards "yes".
APICv can have such a profound impact on performance (and functionality) that
definitively knowing that it's enabled seems justified.