linux-kernel - Re: [PATCH v9 09/43] KVM: arm64: Allow passing machine type in KVM creation

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Message-ID: <e33e6268-a6a5-421d-97c1-b26044e3cfa5@redhat.com>
Date: Tue, 1 Jul 2025 16:38:44 +1000
From: Gavin Shan <gshan@...hat.com>
To: Steven Price <steven.price@....com>, kvm@...r.kernel.org,
 kvmarm@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
 Oliver Upton <oliver.upton@...ux.dev>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Joey Gouly <joey.gouly@....com>,
 Alexandru Elisei <alexandru.elisei@....com>,
 Christoffer Dall <christoffer.dall@....com>, Fuad Tabba <tabba@...gle.com>,
 linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
 Shanker Donthineni <sdonthineni@...dia.com>, Alper Gun
 <alpergun@...gle.com>, "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
 Emi Kisanuki <fj0570is@...itsu.com>
Subject: Re: [PATCH v9 09/43] KVM: arm64: Allow passing machine type in KVM
 creation

On 6/11/25 8:48 PM, Steven Price wrote:
> Previously machine type was used purely for specifying the physical
> address size of the guest. Reserve the higher bits to specify an ARM
> specific machine type and declare a new type 'KVM_VM_TYPE_ARM_REALM'
> used to create a realm guest.
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>
> Reviewed-by: Gavin Shan <gshan@...hat.com>
> Signed-off-by: Steven Price <steven.price@....com>
> ---
> Changes since v7:
>   * Add some documentation explaining the new machine type.
> Changes since v6:
>   * Make the check for kvm_rme_is_available more visible and report an
>     error code of -EPERM (instead of -EINVAL) to make it explicit that
>     the kernel supports RME, but the platform doesn't.
> ---
>   Documentation/virt/kvm/api.rst | 16 ++++++++++++++--
>   arch/arm64/kvm/arm.c           | 15 +++++++++++++++
>   arch/arm64/kvm/mmu.c           |  3 ---
>   include/uapi/linux/kvm.h       | 19 +++++++++++++++----
>   4 files changed, 44 insertions(+), 9 deletions(-)
> 

One nitpick below.

> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 65543289f75c..0049d67fe38f 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -181,8 +181,20 @@ flag KVM_VM_MIPS_VZ.
>   ARM64:
>   ^^^^^^
>   
> -On arm64, the physical address size for a VM (IPA Size limit) is limited
> -to 40bits by default. The limit can be configured if the host supports the
> +On arm64, the machine type identifier is used to encode a type and the
> +physical address size for the VM. The lower byte (bits[7-0]) encode the
> +address size and the upper bits[11-8] encode a machine type. The machine
> +types that might be available are:
> +
> + ======================   ============================================
> + KVM_VM_TYPE_ARM_NORMAL   A standard VM
> + KVM_VM_TYPE_ARM_REALM    A "Realm" VM using the Arm Confidential
> +                          Compute extensions, the VM's memory is
> +                          protected from the host.
> + ======================   ============================================
> +
> +The physical address size for a VM (IPA Size limit) is limited to 40bits
> +by default. The limit can be configured if the host supports the
>   extension KVM_CAP_ARM_VM_IPA_SIZE. When supported, use
>   KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits) to set the size in the machine type
>   identifier, where IPA_Bits is the maximum width of any physical
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 8080443d24af..b3e3323573c6 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -172,6 +172,21 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>   	mutex_unlock(&kvm->lock);
>   #endif
>   
> +	if (type & ~(KVM_VM_TYPE_ARM_MASK | KVM_VM_TYPE_ARM_IPA_SIZE_MASK))
> +		return -EINVAL;
> +
> +	switch (type & KVM_VM_TYPE_ARM_MASK) {
> +	case KVM_VM_TYPE_ARM_NORMAL:
> +		break;
> +	case KVM_VM_TYPE_ARM_REALM:
> +		if (!static_branch_unlikely(&kvm_rme_is_available))
> +			return -EPERM;
> +		kvm->arch.is_realm = true;
> +		break;

Here we need to explicitly set the realm's state to REALM_STATE_NONE even though
it's equal to zero, and the struct vm should have been cleared on allocation.

		WRITE_ONCE(kvm->arch.realm.state, REALM_STATE_NONE);
		kvm->arch.is_realm = true;
		break;

> +	default:
> +		return -EINVAL;
> +	}
> +
>   	kvm_init_nested(kvm);
>   
>   	ret = kvm_share_hyp(kvm, kvm + 1);

[...]

Thanks,
Gavin