Re: [PATCH v6 11/11] arm64: Document Arm Confidential Compute

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 11/10/2024 15:14, Steven Price wrote:
> On 08/10/2024 12:05, Jean-Philippe Brucker wrote:
>> On Fri, Oct 04, 2024 at 03:43:06PM +0100, Steven Price wrote:
>>> Add some documentation on Arm CCA and the requirements for running Linux
>>> as a Realm guest. Also update booting.rst to describe the requirement
>>> for RIPAS RAM.
>>>
>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>> ---
>>>   Documentation/arch/arm64/arm-cca.rst | 67 ++++++++++++++++++++++++++++
>>>   Documentation/arch/arm64/booting.rst |  3 ++
>>>   Documentation/arch/arm64/index.rst   |  1 +
>>>   3 files changed, 71 insertions(+)
>>>   create mode 100644 Documentation/arch/arm64/arm-cca.rst
>>>
>>> diff --git a/Documentation/arch/arm64/arm-cca.rst b/Documentation/arch/arm64/arm-cca.rst
>>> new file mode 100644
>>> index 000000000000..ab7f90e64c2f
>>> --- /dev/null
>>> +++ b/Documentation/arch/arm64/arm-cca.rst
>>> @@ -0,0 +1,67 @@
>>> +.. SPDX-License-Identifier: GPL-2.0
>>> +
>>> +=====================================
>>> +Arm Confidential Compute Architecture
>>> +=====================================
>>> +
>>> +Arm systems that support the Realm Management Extension (RME) contain
>>> +hardware to allow a VM guest to be run in a way which protects the code
>>> +and data of the guest from the hypervisor. It extends the older "two
>>> +world" model (Normal and Secure World) into four worlds: Normal, Secure,
>>> +Root and Realm. Linux can then also be run as a guest to a monitor
>>> +running in the Realm world.
>>> +
>>> +The monitor running in the Realm world is known as the Realm Management
>>> +Monitor (RMM) and implements the Realm Management Monitor
>>> +specification[1]. The monitor acts a bit like a hypervisor (e.g. it runs
>>> +in EL2 and manages the stage 2 page tables etc of the guests running in
>>> +Realm world), however much of the control is handled by a hypervisor
>>> +running in the Normal World. The Normal World hypervisor uses the Realm
>>> +Management Interface (RMI) defined by the RMM specification to request
>>> +the RMM to perform operations (e.g. mapping memory or executing a vCPU).
>>> +
>>> +The RMM defines an environment for guests where the address space (IPA)
>>> +is split into two. The lower half is protected - any memory that is
>>> +mapped in this half cannot be seen by the Normal World and the RMM
>>> +restricts what operations the Normal World can perform on this memory
>>> +(e.g. the Normal World cannot replace pages in this region without the
>>> +guest's cooperation). The upper half is shared, the Normal World is free
>>> +to make changes to the pages in this region, and is able to emulate MMIO
>>> +devices in this region too.
>>> +
>>> +A guest running in a Realm may also communicate with the RMM to request
>>> +changes in its environment or to perform attestation about its
>>> +environment. In particular it may request that areas of the protected
>>> +address space are transitioned between 'RAM' and 'EMPTY' (in either
>>> +direction). This allows a Realm guest to give up memory to be returned
>>> +to the Normal World, or to request new memory from the Normal World.
>>> +Without an explicit request from the Realm guest the RMM will otherwise
>>> +prevent the Normal World from making these changes.
>>
>> We could mention that this interface is "RSI", so readers know what to
>> look for next
> 
> Good idea.
> 
>>> +
>>> +Linux as a Realm Guest
>>> +----------------------
>>> +
>>> +To run Linux as a guest within a Realm, the following must be provided
>>> +either by the VMM or by a `boot loader` run in the Realm before Linux:
>>> +
>>> + * All protected RAM described to Linux (by DT or ACPI) must be marked
>>> +   RIPAS RAM before handing over the Linux.
>>
>> "handing control over to Linux", or something like that?
> 
> Indeed that actually makes grammatical sense! ;)
> 
>>> +
>>> + * MMIO devices must be either unprotected (e.g. emulated by the Normal
>>> +   World) or marked RIPAS DEV.
>>> +
>>> + * MMIO devices emulated by the Normal World and used very early in boot
>>> +   (specifically earlycon) must be specified in the upper half of IPA.
>>> +   For earlycon this can be done by specifying the address on the
>>> +   command line, e.g.: ``earlycon=uart,mmio,0x101000000``
>>
>> This is going to be needed frequently, so maybe we should explain in a
>> little more detail how we come up with this value: "e.g. with an IPA size
>> of 33 and the base address of the emulated UART at 0x1000000,
>> ``earlycon=uart,mmio,0x101000000``"
>>
>> (Because the example IPA size is rather unintuitive and specific to the
>> kvmtool memory map)
> 

With the above addressed:

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>