Subject: Re: [PATCH v10 06/43] arm64: RME: Define the user ABI

From: Marc Zyngier <[email-masked]>
Date: Wed, 01 Oct 2025 13:28:47 +0100

On Wed, 20 Aug 2025 15:55:26 +0100,
Steven Price <[email-masked]> wrote:
> 
> There is one (multiplexed) CAP which can be used to create, populate and
> then activate the realm.
> 
> Co-developed-by: Suzuki K Poulose <[email-masked]>
> Signed-off-by: Suzuki K Poulose <[email-masked]>
> Signed-off-by: Steven Price <[email-masked]>
> Reviewed-by: Gavin Shan <[email-masked]>
> ---
> Changes since v9:
>  * Improvements to documentation.
>  * Bump the magic number for KVM_CAP_ARM_RME to avoid conflicts.
> Changes since v8:
>  * Minor improvements to documentation following review.
>  * Bump the magic numbers to avoid conflicts.
> Changes since v7:
>  * Add documentation of new ioctls
>  * Bump the magic numbers to avoid conflicts
> Changes since v6:
>  * Rename some of the symbols to make their usage clearer and avoid
>    repetition.
> Changes from v5:
>  * Actually expose the new VCPU capability (KVM_ARM_VCPU_REC) by bumping
>    KVM_VCPU_MAX_FEATURES - note this also exposes KVM_ARM_VCPU_HAS_EL2!
> ---
>  Documentation/virt/kvm/api.rst    | 71 +++++++++++++++++++++++++++++++
>  arch/arm64/include/uapi/asm/kvm.h | 49 +++++++++++++++++++++
>  include/uapi/linux/kvm.h          | 10 +++++
>  3 files changed, 130 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 6aa40ee05a4a..69c0a9eba6c5 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -3549,6 +3549,11 @@ Possible features:
>  	  Depends on KVM_CAP_ARM_EL2_E2H0.
>  	  KVM_ARM_VCPU_HAS_EL2 must also be set.
>  
> +	- KVM_ARM_VCPU_REC: Allocate a REC (Realm Execution Context) for this
> +	  VCPU. This must be specified on all VCPUs created in a Realm VM.
> +	  Depends on KVM_CAP_ARM_RME.
> +	  Requires KVM_ARM_VCPU_FINALIZE(KVM_ARM_VCPU_REC).
> +
>  4.83 KVM_ARM_PREFERRED_TARGET
>  -----------------------------
>  
> @@ -5122,6 +5127,7 @@ Recognised values for feature:
>  
>    =====      ===========================================
>    arm64      KVM_ARM_VCPU_SVE (requires KVM_CAP_ARM_SVE)
> +  arm64      KVM_ARM_VCPU_REC (requires KVM_CAP_ARM_RME)
>    =====      ===========================================
>  
>  Finalizes the configuration of the specified vcpu feature.
> @@ -6476,6 +6482,30 @@ the capability to be present.
>  
>  `flags` must currently be zero.
>  
> +4.144 KVM_ARM_VCPU_RMM_PSCI_COMPLETE
> +------------------------------------
> +
> +:Capability: KVM_CAP_ARM_RME
> +:Architectures: arm64
> +:Type: vcpu ioctl
> +:Parameters: struct kvm_arm_rmm_psci_complete (in)
> +:Returns: 0 if successful, < 0 on error
> +
> +::
> +
> +  struct kvm_arm_rmm_psci_complete {
> +	__u64 target_mpidr;
> +	__u32 psci_status;
> +	__u32 padding[3];
> +  };
> +
> +Where PSCI functions are handled by user space, the RMM needs to be informed of
> +the target of the operation using `target_mpidr`, along with the status
> +(`psci_status`). The RMM v1.0 specification defines two functions that require
> +this call: PSCI_CPU_ON and PSCI_AFFINITY_INFO.
> +
> +If the kernel is handling PSCI then this is done automatically and the VMM
> +doesn't need to call this ioctl.

Why should userspace involved in this? Why can't this be a
notification that the host delivers to the RMM when the vcpu is about
to run?

>  
>  .. _kvm_run:
>  
> @@ -8662,6 +8692,47 @@ This capability indicate to the userspace whether a PFNMAP memory region
>  can be safely mapped as cacheable. This relies on the presence of
>  force write back (FWB) feature support on the hardware.
>  
> +7.44 KVM_CAP_ARM_RME
> +--------------------
> +
> +:Architectures: arm64
> +:Target: VM
> +:Parameters: args[0] provides an action, args[1] points to a structure in
> +             memory for the action.
> +:Returns: 0 on success, negative value on error
> +
> +Used to configure and set up the memory for a Realm. The available actions are:
> +
> +================================= =============================================
> + KVM_CAP_ARM_RME_CONFIG_REALM     Takes struct arm_rme_config as args[1] and
> +                                  configures realm parameters prior to it being
> +                                  created.
> +
> +                                  Options are ARM_RME_CONFIG_RPV to set the
> +                                  "Realm Personalization Value" and
> +                                  ARM_RME_CONFIG_HASH_ALGO to set the hash
> +                                  algorithm.
> +
> + KVM_CAP_ARM_RME_CREATE_REALM     Request the RMM to create the realm. The
> +                                  realm's configuration parameters must be set
> +                                  first.
> +
> + KVM_CAP_ARM_RME_INIT_RIPAS_REALM Takes struct arm_rme_init_ripas as args[1]
> +                                  and sets the RIPAS (Realm IPA State) to
> +                                  RIPAS_RAM of a specified area of the realm's
> +                                  IPA.
> +
> + KVM_CAP_ARM_RME_POPULATE_REALM   Takes struct arm_rme_populate_realm as
> +                                  args[1] and populates a region of protected
> +                                  address space by copying the data from the
> +                                  shared alias.
> +
> + KVM_CAP_ARM_RME_ACTIVATE_REALM   Request the RMM to activate the realm. No
> +                                  changes can be made to the Realm's populated
> +                                  memory, IPA state, configuration parameters
> +                                  or vCPU additions after this step.
> +================================= =============================================
> +

These are not capabilities, they are actions that the VMM may perform
on a VM. You don't configure a VM using capabilities. You use it to
buy into some behaviours, but that's all.

And then there is the semantic of this stuff. Why do I need something
like KVM_CAP_ARM_RME_CREATE_REALM when I can just pass this as part of
the VM type? Why do I need a new way to describe memory region when we
already have memslots for that exact purpose?

Overall, you are leaking the RMM interface into userspace, and that's
an absolute show-stopper. We have an API, it is not pretty, but it
exists. We don't need another one that will be just as broken. If the
RMM needs some impedance matching, that's the kernel's job.

	M.

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