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From: Oliver Upton <oliver.upton@linux.dev>
To: Alexandru Elisei <alexandru.elisei@arm.com>
Cc: maz@kernel.org, joey.gouly@arm.com, suzuki.poulose@arm.com,
	yuzenghui@huawei.com, linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.linux.dev, catalin.marinas@arm.com, will@kernel.org
Subject: Re: [PATCH v2 2/2] KVM: arm64: VHE: Save and restore host MDCR_EL2
 value correctly
Message-ID: <aLk38kyxyoe0CYq-@linux.dev>
References: <20250902130833.338216-1-alexandru.elisei@arm.com>
 <20250902130833.338216-3-alexandru.elisei@arm.com>
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In-Reply-To: <20250902130833.338216-3-alexandru.elisei@arm.com>
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On Tue, Sep 02, 2025 at 02:08:33PM +0100, Alexandru Elisei wrote:
> Prior to commit 75a5fbaf6623 ("KVM: arm64: Compute MDCR_EL2 at
> vcpu_load()"), host MDCR_EL2 was saved correctly:
> 
> kvm_arch_vcpu_load()
>   kvm_vcpu_load_debug() /* Doesn't touch hardware MDCR_EL2. */
>   kvm_vcpu_load_vhe()
>     __activate_traps_common()
>        /* Saves host MDCR_EL2. */
>        *host_data_ptr(host_debug_state.mdcr_el2) = read_sysreg(mdcr_el2)
>        /* Writes VCPU MDCR_EL2. */
>        write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2)
> 
> The MDCR_EL2 value saved previously was restored in
> kvm_arch_vcpu_put() -> kvm_vcpu_put_vhe().
> 
> After the aforementioned commit, host MDCR_EL2 is never saved:
> 
> kvm_arch_vcpu_load()
>   kvm_vcpu_load_debug() /* Writes VCPU MDCR_EL2 */
>   kvm_vcpu_load_vhe()
>     __activate_traps_common()
>        /* Saves **VCPU** MDCR_EL2. */
>        *host_data_ptr(host_debug_state.mdcr_el2) = read_sysreg(mdcr_el2)
>        /* Writes VCPU MDCR_EL2 a second time. */
>        write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2)
> 
> kvm_arch_vcpu_put() -> kvm_vcpu_put_vhe() then restores the VCPU MDCR_EL2
> value. Also VCPU's MDCR_EL2 value gets written to hardware twice now.
> 
> Fix this by saving the host MDCR_EL2 in kvm_arch_vcpu_load() before it gets
> overwritten by the VCPU's MDCR_EL2 value, and restore it on VCPU put.
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>

Thanks,
Oliver