[RFC] Support for Arm CCA VMs on Linux

Suzuki K Poulose
2023-01-27T11:33:34Z

We are happy to announce the early RFC version of the Arm
Confidential Compute Architecture (CCA) support for the Linux
stack. The intention is to seek early feedback in the following areas:
* KVM integration of the Arm CCA
* KVM UABI for managing the Realms, seeking to generalise the operations
wherever possible with other Confidential Compute solutions.
Note: This version doesn't support Guest Private memory, which will be added
later (see below).
* Linux Guest support for Realms

Arm CCA Introduction
=====================

The Arm CCA is a reference software architecture and implementation that builds
on the Realm Management Extension (RME), enabling the execution of Virtual
machines, while preventing access by more privileged software, such as hypervisor.
The Arm CCA allows the hypervisor to control the VM, but removes the right for
access to the code, register state or data that is used by VM.
More information on the architecture is available here[0].

Arm CCA Reference Software Architecture

Realm World || Normal World || Secure World ||
|| | || ||
EL0 x-------x || x----x | x------x || ||
| Realm | || | | | | | || ||
| | || | VM | | | | || ||
----| VM* |---------||-| |---| |-||----------------||
| | || | | | | H | || ||
EL1 x-------x || x----x | | | || ||
^ || | | o | || ||
| || | | | || ||
------- R*------------------------| s -|---------------------
S || | | || ||
I || | t | || ||
| || | | || ||
v || x------x || ||
EL2 RMM* || ^ || ||
^ || | || ||
========|=============================|========================
| | SMC
x--------- *RMI* -------------x

EL3 Root World
EL3 Firmware
===============================================================
Where :
RMM - Realm Management Monitor
RMI - Realm Management Interface
RSI - Realm Service Interface
SMC - Secure Monitor Call

RME introduces a new security state "Realm world", in addition to the
traditional Secure and Non-Secure states. The Arm CCA defines a new component,
Realm Management Monitor (RMM) that runs at R-EL2. This is a standard piece of
firmware, verified, installed and loaded by the EL3 firmware (e.g, TF-A), at
system boot.

The RMM provides standard interfaces - Realm Management Interface (RMI) - to the
Normal world hypervisor to manage the VMs running in the Realm world (also called
Realms in short). These are exposed via SMC and are routed through the EL3
firmwre.
The RMI interface includes:
- Move a physical page from the Normal world to the Realm world
- Creating a Realm with requested parameters, tracked via Realm Descriptor (RD)
- Creating VCPUs aka Realm Execution Context (REC), with initial register state.
- Create stage2 translation table at any level.
- Load initial images into Realm Memory from normal world memory
- Schedule RECs (vCPUs) and handle exits
- Inject virtual interrupts into the Realm
- Service stage2 runtime faults with pages (provided by host, scrubbed by RMM).
- Create "shared" mappings that can be accessed by VMM/Hyp.
- Reclaim the memory allocated for the RAM and RTTs (Realm Translation Tables)

However v1.0 of RMM specifications doesn't support:
- Paging protected memory of a Realm VM. Thus the pages backing the protected
memory region must be pinned.
- Live migration of Realms.
- Trusted Device assignment.
- Physical interrupt backed Virtual interrupts for Realms

RMM also provides certain services to the Realms via SMC, called Realm Service
Interface (RSI). These include:
- Realm Guest Configuration.
- Attestation & Measurement services
- Managing the state of an Intermediate Physical Address (IPA aka GPA) page.
- Host Call service (Communication with the Normal world Hypervisor)

The specifications for the RMM software is currently at *v1.0-Beta2* and the
latest version is available here [1].

The Trusted Firmware foundation has an implementation of the RMM - TF-RMM -
available here [3].

Implementation
=================

This version of the stack is based on the RMM specification v1.0-Beta0[2], with
following exceptions :
- TF-RMM/KVM currently doesn't support the optional features of PMU,
SVE and Self-hosted debug (coming soon).
- The RSI_HOST_CALL structure alignment requirement is reduced to match
RMM v1.0 Beta1
- RMI/RSI version numbers do not match the RMM spec. This will be
resolved once the spec/implementation is complete, across TF-RMM+Linux stack.

We plan to update the stack to support the latest version of the RMMv1.0 spec
in the coming revisions.

This release includes the following components :

a) Linux Kernel
i) Host / KVM support - Support for driving the Realms via RMI. This is
dependent on running in the Kernel at EL2 (aka VHE mode). Also provides
UABI for VMMs to manage the Realm VMs. The support is restricted to 4K page
size, matching the Stage2 granule supported by RMM. The VMM is responsible
for making sure the guest memory is locked.

TODO: Guest Private memory[10] integration - We have been following the
series and support will be added once it is merged upstream.

ii) Guest support - Support for a Linux Kernel to run in the Realm VM at
Realm-EL1, using RSI services. This includes virtio support (virtio-v1.0
only). All I/O are treated as non-secure/shared.

c) kvmtool - VMM changes required to manage Realm VMs. No guest private memory
as mentioned above.
d) kvm-unit-tests - Support for running in Realms along with additional tests
for RSI ABI.

Running the stack
====================

To run/test the stack, you would need the following components :

1) FVP Base AEM RevC model with FEAT_RME support [4]
2) TF-A firmware for EL3 [5]
3) TF-A RMM for R-EL2 [3]
4) Linux Kernel [6]
5) kvmtool [7]
6) kvm-unit-tests [8]

Instructions for building the firmware components and running the model are
available here [9]. Once, the host kernel is booted, a Realm can be launched by
invoking the `lkvm` commad as follows:

$ lkvm run --realm \
--measurement-algo=["sha256", "sha512"] \
--disable-sve \
<normal-vm-options>

Where:
* --measurement-algo (Optional) specifies the algorithm selected for creating the
initial measurements by the RMM for this Realm (defaults to sha256).
* GICv3 is mandatory for the Realms.
* SVE is not yet supported in the TF-RMM, and thus must be disabled using
--disable-sve

You may also run the kvm-unit-tests inside the Realm world, using the similar
options as above.

Links
============

[0] Arm CCA Landing page (See Key Resources section for various documentations)
https://www.arm.com/architecture/security-features/arm-confidential-compute-architecture

[1] RMM Specification Latest
https://developer.arm.com/documentation/den0137/latest

[2] RMM v1.0-Beta0 specification
https://developer.arm.com/documentation/den0137/1-0bet0/

[3] Trusted Firmware RMM - TF-RMM
https://www.trustedfirmware.org/projects/tf-rmm/
GIT: https://git.trustedfirmware.org/TF-RMM/tf-rmm.git

[4] FVP Base RevC AEM Model (available on x86_64 / Arm64 Linux)
https://developer.arm.com/Tools%20and%20Software/Fixed%20Virtual%20Platforms

[5] Trusted Firmware for A class
https://www.trustedfirmware.org/projects/tf-a/

[6] Linux kernel support for Arm-CCA
https://gitlab.arm.com/linux-arm/linux-cca
Host Support branch: cca-host/rfc-v1
Guest Support branch: cca-guest/rfc-v1

[7] kvmtool support for Arm CCA
https://gitlab.arm.com/linux-arm/kvmtool-cca cca/rfc-v1

[8] kvm-unit-tests support for Arm CCA
https://gitlab.arm.com/linux-arm/kvm-unit-tests-cca cca/rfc-v1

[9] Instructions for Building Firmware components and running the model, see
section 4.19.2 "Building and running TF-A with RME"
https://trustedfirmware-a.readthedocs.io/en/latest/components/realm-management-extension.html#building-and-running-tf-a-with-rme

[10] fd based Guest Private memory for KVM
https://lkml.kernel.org/r/20221202061347.1070246-1-chao.p.peng@linux.intel.com

Cc: Alexandru Elisei <alexandru.elisei@arm.com>
Cc: Andrew Jones <andrew.jones@linux.dev>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chao Peng <chao.p.peng@linux.intel.com>
Cc: Christoffer Dall <christoffer.dall@arm.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: James Morse <james.morse@arm.com>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
Cc: Joey Gouly <Joey.Gouly@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Quentin Perret <qperret@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Thomas Huth <thuth@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Zenghui Yu <yuzenghui@huawei.com>
To: linux-coco@lists.linux.dev
To: kvmarm@lists.linux.dev
Cc: kvmarm@lists.cs.columbia.edu
Cc: linux-arm-kernel@lists.infradead.org
To: linux-kernel@vger.kernel.org
To: kvm@vger.kernel.org