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Add some initial documentation for pKVM to help people understand what is supported, the limitations of protected VMs when compared to non-protected VMs and also what is left to do. Reviewed-by: Fuad Tabba <tabba@google.com> Tested-by: Fuad Tabba <tabba@google.com> Tested-by: Mostafa Saleh <smostafa@google.com> Signed-off-by: Will Deacon <will@kernel.org> Link: https://patch.msgid.link/20260330144841.26181-33-will@kernel.org Signed-off-by: Marc Zyngier <maz@kernel.org>
107 lines
3.9 KiB
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107 lines
3.9 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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====================
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Protected KVM (pKVM)
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====================
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**NOTE**: pKVM is currently an experimental, development feature and
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subject to breaking changes as new isolation features are implemented.
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Please reach out to the developers at kvmarm@lists.linux.dev if you have
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any questions.
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Overview
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========
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Booting a host kernel with '``kvm-arm.mode=protected``' enables
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"Protected KVM" (pKVM). During boot, pKVM installs a stage-2 identity
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map page-table for the host and uses it to isolate the hypervisor
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running at EL2 from the rest of the host running at EL1/0.
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pKVM permits creation of protected virtual machines (pVMs) by passing
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the ``KVM_VM_TYPE_ARM_PROTECTED`` machine type identifier to the
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``KVM_CREATE_VM`` ioctl(). The hypervisor isolates pVMs from the host by
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unmapping pages from the stage-2 identity map as they are accessed by a
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pVM. Hypercalls are provided for a pVM to share specific regions of its
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IPA space back with the host, allowing for communication with the VMM.
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A Linux guest must be configured with ``CONFIG_ARM_PKVM_GUEST=y`` in
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order to issue these hypercalls.
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See hypercalls.rst for more details.
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Isolation mechanisms
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====================
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pKVM relies on a number of mechanisms to isolate PVMs from the host:
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CPU memory isolation
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--------------------
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Status: Isolation of anonymous memory and metadata pages.
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Metadata pages (e.g. page-table pages and '``struct kvm_vcpu``' pages)
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are donated from the host to the hypervisor during pVM creation and
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are consequently unmapped from the stage-2 identity map until the pVM is
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destroyed.
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Similarly to regular KVM, pages are lazily mapped into the guest in
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response to stage-2 page faults handled by the host. However, when
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running a pVM, these pages are first pinned and then unmapped from the
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stage-2 identity map as part of the donation procedure. This gives rise
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to some user-visible differences when compared to non-protected VMs,
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largely due to the lack of MMU notifiers:
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* Memslots cannot be moved or deleted once the pVM has started running.
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* Read-only memslots and dirty logging are not supported.
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* With the exception of swap, file-backed pages cannot be mapped into a
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pVM.
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* Donated pages are accounted against ``RLIMIT_MLOCK`` and so the VMM
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must have a sufficient resource limit or be granted ``CAP_IPC_LOCK``.
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The lack of a runtime reclaim mechanism means that memory locked for
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a pVM will remain locked until the pVM is destroyed.
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* Changes to the VMM address space (e.g. a ``MAP_FIXED`` mmap() over a
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mapping associated with a memslot) are not reflected in the guest and
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may lead to loss of coherency.
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* Accessing pVM memory that has not been shared back will result in the
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delivery of a SIGSEGV.
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* If a system call accesses pVM memory that has not been shared back
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then it will either return ``-EFAULT`` or forcefully reclaim the
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memory pages. Reclaimed memory is zeroed by the hypervisor and a
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subsequent attempt to access it in the pVM will return ``-EFAULT``
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from the ``VCPU_RUN`` ioctl().
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CPU state isolation
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-------------------
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Status: **Unimplemented.**
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DMA isolation using an IOMMU
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----------------------------
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Status: **Unimplemented.**
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Proxying of Trustzone services
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------------------------------
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Status: FF-A and PSCI calls from the host are proxied by the pKVM
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hypervisor.
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The FF-A proxy ensures that the host cannot share pVM or hypervisor
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memory with Trustzone as part of a "confused deputy" attack.
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The PSCI proxy ensures that CPUs always have the stage-2 identity map
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installed when they are executing in the host.
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Protected VM firmware (pvmfw)
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-----------------------------
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Status: **Unimplemented.**
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Resources
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=========
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Quentin Perret's KVM Forum 2022 talk entitled "Protected KVM on arm64: A
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technical deep dive" remains a good resource for learning more about
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pKVM, despite some of the details having changed in the meantime:
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https://www.youtube.com/watch?v=9npebeVFbFw
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