Re: [PATCH v3 26/27] KVM: arm64: Move HCRX_EL2 switch to load/put on VHE systems

From: Eric Auger <eric.auger@redhat.com>
To: Marc Zyngier <maz@kernel.org>,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Mark Brown <broonie@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Andre Przywara <andre.przywara@arm.com>,
	Chase Conklin <chase.conklin@arm.com>,
	Ganapatrao Kulkarni <gankulkarni@os.amperecomputing.com>,
	Darren Hart <darren@os.amperecomputing.com>,
	Miguel Luis <miguel.luis@oracle.com>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Zenghui Yu <yuzenghui@huawei.com>
Subject: Re: [PATCH v3 26/27] KVM: arm64: Move HCRX_EL2 switch to load/put on VHE systems
Date: Thu, 10 Aug 2023 14:38:26 +0200	[thread overview]
Message-ID: <15fc525c-7357-7457-c3c2-a8211619e787@redhat.com> (raw)
In-Reply-To: <20230808114711.2013842-27-maz@kernel.org>

Hi Marc,

On 8/8/23 13:47, Marc Zyngier wrote:
> Although the nVHE behaviour requires HCRX_EL2 to be switched
> on each switch between host and guest, there is nothing in
> this register that would affect a VHE host.
>
> It is thus possible to save/restore this register on load/put
> on VHE systems, avoiding unnecessary sysreg access on the hot
> path. Additionally, it avoids unnecessary traps when running
> with NV.
>
> To achieve this, simply move the read/writes to the *_common()
> helpers, which are called on load/put on VHE, and more eagerly
> on nVHE.
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/hyp/include/hyp/switch.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index a4750070563f..060c5a0409e5 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -197,6 +197,9 @@ static inline void __activate_traps_common(struct kvm_vcpu *vcpu)
>  	vcpu->arch.mdcr_el2_host = read_sysreg(mdcr_el2);
>  	write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2);
>  
> +	if (cpus_have_final_cap(ARM64_HAS_HCX))
> +		write_sysreg_s(HCRX_GUEST_FLAGS, SYS_HCRX_EL2);
> +
>  	__activate_traps_hfgxtr(vcpu);
>  }
>  
> @@ -213,6 +216,9 @@ static inline void __deactivate_traps_common(struct kvm_vcpu *vcpu)
>  		vcpu_clear_flag(vcpu, PMUSERENR_ON_CPU);
>  	}
>  
> +	if (cpus_have_final_cap(ARM64_HAS_HCX))
> +		write_sysreg_s(HCRX_HOST_FLAGS, SYS_HCRX_EL2);
> +
>  	__deactivate_traps_hfgxtr(vcpu);
>  }
>  
> @@ -227,9 +233,6 @@ static inline void ___activate_traps(struct kvm_vcpu *vcpu)
>  
>  	if (cpus_have_final_cap(ARM64_HAS_RAS_EXTN) && (hcr & HCR_VSE))
>  		write_sysreg_s(vcpu->arch.vsesr_el2, SYS_VSESR_EL2);
> -
> -	if (cpus_have_final_cap(ARM64_HAS_HCX))
> -		write_sysreg_s(HCRX_GUEST_FLAGS, SYS_HCRX_EL2);
>  }
>  
>  static inline void ___deactivate_traps(struct kvm_vcpu *vcpu)
> @@ -244,9 +247,6 @@ static inline void ___deactivate_traps(struct kvm_vcpu *vcpu)
>  		vcpu->arch.hcr_el2 &= ~HCR_VSE;
>  		vcpu->arch.hcr_el2 |= read_sysreg(hcr_el2) & HCR_VSE;
>  	}
> -
> -	if (cpus_have_final_cap(ARM64_HAS_HCX))
> -		write_sysreg_s(HCRX_HOST_FLAGS, SYS_HCRX_EL2);
>  }
>  
>  static inline bool __populate_fault_info(struct kvm_vcpu *vcpu)
Reviewed-by: Eric Auger <eric.auger@redhat.com>

Thanks

Eric

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