Re: [PATCH v12 06/46] arm64: RMI: Define the user ABI

[Marc Zyngier <maz@kernel.org>]

On Mon, 02 Mar 2026 17:13:41 +0000,
Suzuki K Poulose <suzuki.poulose@arm.com> wrote:
> 
> On 02/03/2026 15:23, Steven Price wrote:
> > Hi Marc,
> > 
> > On 02/03/2026 14:25, Marc Zyngier wrote:
> >> On Wed, 17 Dec 2025 10:10:43 +0000,
> >> Steven Price <steven.price@arm.com> wrote:
> >>> 
> >>> There is one CAP which identified the presence of CCA, and two ioctls.
> >>> One ioctl is used to populate memory and the other is used when user
> >>> space is providing the PSCI implementation to identify the target of the
> >>> operation.
> >>> 
> >>> Signed-off-by: Steven Price <steven.price@arm.com>
> >>> ---
> >>> Changes since v11:
> >>>   * Completely reworked to be more implicit. Rather than having explicit
> >>>     CAP operations to progress the realm construction these operations
> >>>     are done when needed (on populating and on first vCPU run).
> >>>   * Populate and PSCI complete are promoted to proper ioctls.
> >>> Changes since v10:
> >>>   * Rename symbols from RME to RMI.
> >>> 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 | 57 ++++++++++++++++++++++++++++++++++
> >>>   include/uapi/linux/kvm.h       | 23 ++++++++++++++
> >>>   2 files changed, 80 insertions(+)
> >>> 
> >>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> >>> index 01a3abef8abb..2d5dc7e48954 100644
> >>> --- a/Documentation/virt/kvm/api.rst
> >>> +++ b/Documentation/virt/kvm/api.rst
> >>> @@ -6517,6 +6517,54 @@ the capability to be present.
> >>>     `flags` must currently be zero.
> >>>   +4.144 KVM_ARM_VCPU_RMI_PSCI_COMPLETE
> >>> +------------------------------------
> >>> +
> >>> +:Capability: KVM_CAP_ARM_RMI
> >>> +:Architectures: arm64
> >>> +:Type: vcpu ioctl
> >>> +:Parameters: struct kvm_arm_rmi_psci_complete (in)
> >>> +:Returns: 0 if successful, < 0 on error
> >>> +
> >>> +::
> >>> +
> >>> +  struct kvm_arm_rmi_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.
> >> 
> >> Shouldn't we make handling of PSCI mandatory for VMMs that deal with
> >> CCA? I suspect it would simplify the implementation significantly.
> > 
> > What do you mean by making it "mandatory for VMMs"? If you mean PSCI is
> > always forwarded to user space then I don't think it's going to make
> > much difference. Patch 27 handles the PSCI changes (72 lines added), and
> > some of that is adding this uAPI for the VMM to handle it.
> > 
> > Removing the functionality to allow the VMM to handle it would obviously
> > simplify things a bit (we can drop this uAPI), but I think the desire is
> > to push this onto user space.
> > 
> >> What vcpu fd does this apply to? The vcpu calling the PSCI function?
> >> Or the target? This is pretty important for PSCI_ON. My guess is that
> >> this is setting the return value for the caller?
> > 
> > Yes the fd is the vcpu calling PSCI. As you say, this is for the return
> > value to be set correctly.
> > 
> >> Assuming this is indeed for the caller, why do we have a different
> >> flow from anything else that returns a result from a hypercall?
> > 
> > I'm not entirely sure what you are suggesting. Do you mean why are we
> > not just writing to the GPRS that would contain the result? The issue
> > here is that the RMM needs to know the PA of the target REC structure -
> > this isn't a return to the guest, but information for the RMM itself to
> > complete the PSCI call.
> > 
> > Ultimately even in the case where the VMM is handling PSCI, it's
> > actually a combination of the VMM and the RMM - with the RMM validating
> > the responses.
> > 
> 
> More importantly, we have to make sure that the "RMI_PSCI_COMPLETE" is
> invoked before both of the following:
>   1. The "source" vCPU is run again
>   2. More importantly the "target" vCPU is run.

I don't understand why (1) is required. Once the VMM gets the request,
the target vcpu can run, and can itself do the completion, without any
additional userspace involvement.

	M.

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