Re: [PATCH v16 16/45] KVM: arm64: CCA: Allow passing the machine type in KVM creation

[Gavin Shan <gshan@redhat.com>]

Hi Steve,

On 8/3/26 11:43 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.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> Changes since v15:
>   * Specify the encoding for each VM type in the documentation.
> Changes since v13:
>   * Rework to use the two top bits for the machine type now that pKVM has
>     merged and used the top bit for KVM_VM_TYPE_ARM_PROTECTED.
>   * Update the documentation to include KVM_VM_TYPE_ARM_PROTECTED as
>     well.
> Changes since v9:
>   * Explictly set realm.state to REALM_STATE_NONE rather than rely on the
>     zeroing of the structure.
> 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 | 21 +++++++++++++++++++--
>   arch/arm64/kvm/arm.c           | 11 +++++++++++
>   include/uapi/linux/kvm.h       |  7 ++++++-
>   3 files changed, 36 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index ce588c05b26a..802601eaf560 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -181,8 +181,25 @@ 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]) encodes the
> +address size and the upper bits[31:30] encode a machine type. The machine
> +types that might be available are:
> +
> + =========================   =======   ========================================
> + Type                        [31:30]   Description
> + =========================   =======   ========================================
> + KVM_VM_TYPE_ARM_NORMAL      0b00      A standard VM
> + KVM_VM_TYPE_ARM_REALM       0b01      A "realm" VM using the Arm Confidential
> +                                       Compute extensions, the VM's memory is
> +                                       protected from the host.
> + KVM_VM_TYPE_ARM_PROTECTED   0b10      A "protected" VM using pKVM to
> +                                       isolate the VM from the host.
> + Reserved                    0b11      Reserved for future use
> + =========================   =======   ========================================
> +
> +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 6f39831dcf5d..9881fd6c511c 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -249,6 +249,17 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>   	mutex_unlock(&kvm->lock);
>   #endif
>   
> +	if ((type & KVM_VM_TYPE_ARM_PROTECTED) &&
> +	    (type & KVM_VM_TYPE_ARM_REALM))
> +		return -EINVAL;
> +
> +	if (type & KVM_VM_TYPE_ARM_REALM) {
> +		if (!static_branch_unlikely(&kvm_rmi_is_available))
> +			return -EINVAL;
> +		kvm_set_realm_state(kvm, REALM_STATE_NONE);
> +		kvm->arch.is_realm = true;
> +	}
> +

Shall we use kvm_arm_rmi_supported() here? In that way, the check is consistent
to what has been done for KVM_CAP_ARM_RMI. You know that a misbehaving VMM can
issue KVM_CREATE_VM to create a realm VM without checking if KVM_CAP_ARM_RMI
is supported by the host.

	if (type & KVM_VM_TYPE_ARM_REALM) {
		if (!kvm_arm_rmi_supported())
			return -EINVAL;

		kvm_set_realm_state(kvm, REALM_STATE_NONE);
		WRITE_ONCE(kvm->arch.is_realm, true);
	}

>   	kvm_init_nested(kvm);
>   
>   	ret = kvm_share_hyp(kvm, kvm + 1);
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 0231ff174a50..adee3936d6ae 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -700,14 +700,19 @@ struct kvm_enable_cap {
>    * address size for the VM. Bits[7-0] are reserved for the guest
>    * PA size shift (i.e, log2(PA_Size)). For backward compatibility,
>    * value 0 implies the default IPA size, 40bits.
> + *
> + * Bits[30-31] are reserved for the VM type
>    */
>   #define KVM_VM_TYPE_ARM_IPA_SIZE_MASK	0xffULL
>   #define KVM_VM_TYPE_ARM_IPA_SIZE(x)		\
>   	((x) & KVM_VM_TYPE_ARM_IPA_SIZE_MASK)
>   
> +#define KVM_VM_TYPE_ARM_NORMAL		0
> +#define KVM_VM_TYPE_ARM_REALM		(1UL << 30)
>   #define KVM_VM_TYPE_ARM_PROTECTED	(1UL << 31)
>   #define KVM_VM_TYPE_ARM_MASK		(KVM_VM_TYPE_ARM_IPA_SIZE_MASK | \
> -					 KVM_VM_TYPE_ARM_PROTECTED)
> +					 KVM_VM_TYPE_ARM_PROTECTED | \
> +					 KVM_VM_TYPE_ARM_REALM)
>   
>   /*
>    * ioctls for /dev/kvm fds:

Thanks,
Gavin