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Date: Fri, 2 Feb 2024 22:00:40 +0000
From: Oliver Upton <oliver.upton@linux.dev>
To: James Clark <james.clark@arm.com>
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	kvmarm@lists.linux.dev, broonie@kernel.org, maz@kernel.org,
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Subject: Re: [PATCH v4 2/7] arm64: KVM: Use shared area to pass PMU event
 state to hypervisor
Message-ID: <Zb1mCCi13AJ_YjFZ@linux.dev>
References: <20240104162714.1062610-1-james.clark@arm.com>
 <20240104162714.1062610-3-james.clark@arm.com>
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In-Reply-To: <20240104162714.1062610-3-james.clark@arm.com>

On Thu, Jan 04, 2024 at 04:27:02PM +0000, James Clark wrote:

[...]

> diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> index c50f8459e4fc..89147a9dc38c 100644
> --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> @@ -130,13 +130,18 @@ static void __hyp_vgic_restore_state(struct kvm_vcpu *vcpu)
>  	}
>  }
>  
> +static struct kvm_pmu_events *kvm_nvhe_get_pmu_events(struct kvm_vcpu *vcpu)
> +{
> +	return &kvm_host_global_state[vcpu->cpu].pmu_events;
> +}
> +
>  /*
>   * Disable host events, enable guest events
>   */
>  #ifdef CONFIG_HW_PERF_EVENTS
>  static bool __pmu_switch_to_guest(struct kvm_vcpu *vcpu)
>  {
> -	struct kvm_pmu_events *pmu = &vcpu->arch.pmu.events;
> +	struct kvm_pmu_events *pmu = kvm_nvhe_get_pmu_events(vcpu);
>  
>  	if (pmu->events_host)
>  		write_sysreg(pmu->events_host, pmcntenclr_el0);
> @@ -152,7 +157,7 @@ static bool __pmu_switch_to_guest(struct kvm_vcpu *vcpu)
>   */
>  static void __pmu_switch_to_host(struct kvm_vcpu *vcpu)
>  {
> -	struct kvm_pmu_events *pmu = &vcpu->arch.pmu.events;
> +	struct kvm_pmu_events *pmu = kvm_nvhe_get_pmu_events(vcpu);
>  
>  	if (pmu->events_guest)
>  		write_sysreg(pmu->events_guest, pmcntenclr_el0);

This now allows the host to program event counters for a protected
guest. That _might_ be a useful feature behind some debug option, but is
most definitely *not* something we want to do for pVMs generally.

Do we even need to make this shared data work at all for pKVM? The rest
of the shared data between pKVM and the kernel is system information,
which (importantly) doesn't have any guest context in it.

I'm perfectly happy leaving these sorts of features broken for pKVM and
using the 'normal' way of getting percpu data to the nVHE hypervisor
otherwise.

-- 
Thanks,
Oliver