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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. Reviewed-by: Gavin Shan <gshan@redhat.com> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com> Signed-off-by: Steven Price <steven.price@arm.com> Link: https://lore.kernel.org/r/20241017131434.40935-12-steven.price@arm.com Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
70 lines
3.4 KiB
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70 lines
3.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=====================================
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Arm Confidential Compute Architecture
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=====================================
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Arm systems that support the Realm Management Extension (RME) contain
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hardware to allow a VM guest to be run in a way which protects the code
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and data of the guest from the hypervisor. It extends the older "two
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world" model (Normal and Secure World) into four worlds: Normal, Secure,
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Root and Realm. Linux can then also be run as a guest to a monitor
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running in the Realm world.
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The monitor running in the Realm world is known as the Realm Management
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Monitor (RMM) and implements the Realm Management Monitor
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specification[1]. The monitor acts a bit like a hypervisor (e.g. it runs
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in EL2 and manages the stage 2 page tables etc of the guests running in
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Realm world), however much of the control is handled by a hypervisor
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running in the Normal World. The Normal World hypervisor uses the Realm
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Management Interface (RMI) defined by the RMM specification to request
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the RMM to perform operations (e.g. mapping memory or executing a vCPU).
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The RMM defines an environment for guests where the address space (IPA)
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is split into two. The lower half is protected - any memory that is
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mapped in this half cannot be seen by the Normal World and the RMM
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restricts what operations the Normal World can perform on this memory
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(e.g. the Normal World cannot replace pages in this region without the
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guest's cooperation). The upper half is shared, the Normal World is free
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to make changes to the pages in this region, and is able to emulate MMIO
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devices in this region too.
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A guest running in a Realm may also communicate with the RMM using the
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Realm Services Interface (RSI) to request changes in its environment or
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to perform attestation about its environment. In particular it may
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request that areas of the protected address space are transitioned
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between 'RAM' and 'EMPTY' (in either direction). This allows a Realm
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guest to give up memory to be returned to the Normal World, or to
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request new memory from the Normal World. Without an explicit request
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from the Realm guest the RMM will otherwise prevent the Normal World
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from making these changes.
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Linux as a Realm Guest
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----------------------
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To run Linux as a guest within a Realm, the following must be provided
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either by the VMM or by a `boot loader` run in the Realm before Linux:
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* All protected RAM described to Linux (by DT or ACPI) must be marked
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RIPAS RAM before handing control over to Linux.
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* MMIO devices must be either unprotected (e.g. emulated by the Normal
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World) or marked RIPAS DEV.
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* MMIO devices emulated by the Normal World and used very early in boot
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(specifically earlycon) must be specified in the upper half of IPA.
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For earlycon this can be done by specifying the address on the
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command line, e.g. with an IPA size of 33 bits and the base address
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of the emulated UART at 0x1000000: ``earlycon=uart,mmio,0x101000000``
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* Linux will use bounce buffers for communicating with unprotected
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devices. It will transition some protected memory to RIPAS EMPTY and
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expect to be able to access unprotected pages at the same IPA address
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but with the highest valid IPA bit set. The expectation is that the
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VMM will remove the physical pages from the protected mapping and
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provide those pages as unprotected pages.
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References
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----------
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[1] https://developer.arm.com/documentation/den0137/
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