Re: [PATCH v1 1/2] KVM: arm64: PMU: Restore the host's PMUSERENR_EL0

From: Marc Zyngier <maz@kernel.org>
To: Reiji Watanabe <reijiw@google.com>
Cc: Oliver Upton <oliver.upton@linux.dev>,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ricardo Koller <ricarkol@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>,
	Will Deacon <will@kernel.org>
Subject: Re: [PATCH v1 1/2] KVM: arm64: PMU: Restore the host's PMUSERENR_EL0
Date: Wed, 29 Mar 2023 08:31:24 +0100	[thread overview]
Message-ID: <87v8ikqaib.wl-maz@kernel.org> (raw)
In-Reply-To: <20230329002136.2463442-2-reijiw@google.com>

On Wed, 29 Mar 2023 01:21:35 +0100,
Reiji Watanabe <reijiw@google.com> wrote:
> 
> Restore the host's PMUSERENR_EL0 value instead of clearing it,
> before returning back to userspace, as the host's EL0 might have
> a direct access to PMU registers (some bits of PMUSERENR_EL0
> might not be zero).
> 
> Fixes: 83a7a4d643d3 ("arm64: perf: Enable PMU counter userspace access for perf event")
> Signed-off-by: Reiji Watanabe <reijiw@google.com>
> ---
>  arch/arm64/include/asm/kvm_host.h       | 3 +++
>  arch/arm64/kvm/hyp/include/hyp/switch.h | 3 ++-
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index bcd774d74f34..82220ecec10e 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -544,6 +544,9 @@ struct kvm_vcpu_arch {
>  
>  	/* Per-vcpu CCSIDR override or NULL */
>  	u32 *ccsidr;
> +
> +	/* the value of host's pmuserenr_el0 before guest entry */
> +	u64	host_pmuserenr_el0;

I don't think we need this in each and every vcpu. Why can't this be
placed in struct kvm_host_data and accessed via the per-cpu pointer?
Maybe even use the PMUSERNR_EL0 field in the sysreg array?

There is probably a number of things that we could move there, but
let's start by not adding more unnecessary stuff to the vcpu
structure.

>  };
>  
>  /*
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index 07d37ff88a3f..44b84fbdde0d 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -82,6 +82,7 @@ static inline void __activate_traps_common(struct kvm_vcpu *vcpu)
>  	 */
>  	if (kvm_arm_support_pmu_v3()) {
>  		write_sysreg(0, pmselr_el0);
> +		vcpu->arch.host_pmuserenr_el0 = read_sysreg(pmuserenr_el0);
>  		write_sysreg(ARMV8_PMU_USERENR_MASK, pmuserenr_el0);
>  	}
>  
> @@ -106,7 +107,7 @@ static inline void __deactivate_traps_common(struct kvm_vcpu *vcpu)
>  
>  	write_sysreg(0, hstr_el2);
>  	if (kvm_arm_support_pmu_v3())
> -		write_sysreg(0, pmuserenr_el0);
> +		write_sysreg(vcpu->arch.host_pmuserenr_el0, pmuserenr_el0);
>  
>  	if (cpus_have_final_cap(ARM64_SME)) {
>  		sysreg_clear_set_s(SYS_HFGRTR_EL2, 0,

Thanks,

	M.

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