Commit 287c6981 authored by Will Deacon's avatar Will Deacon Committed by Marc Zyngier
Browse files

KVM: arm64: Add some initial documentation for pKVM 



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: default avatarFuad Tabba <tabba@google.com>
Tested-by: default avatarFuad Tabba <tabba@google.com>
Tested-by: default avatarMostafa Saleh <smostafa@google.com>
Signed-off-by: default avatarWill Deacon <will@kernel.org>
Link: https://patch.msgid.link/20260330144841.26181-33-will@kernel.org


Signed-off-by: default avatarMarc Zyngier <maz@kernel.org>
parent 8800dbf6

Documentation/admin-guide/kernel-parameters.txt

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@@ -3247,8 +3247,8 @@ Kernel parameters 
			for the host. To force nVHE on VHE hardware, add

			"arm64_sw.hvhe=0 id_aa64mmfr1.vh=0" to the

			command-line.

			"nested" is experimental and should be used with

			extreme caution.

			"nested" and "protected" are experimental and should be

			used with extreme caution.



	kvm-arm.vgic_v3_group0_trap=

			[KVM,ARM,EARLY] Trap guest accesses to GICv3 group-0

Documentation/virt/kvm/arm/index.rst

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@@ -10,6 +10,7 @@ ARM 
   fw-pseudo-registers

   hyp-abi

   hypercalls

   pkvm

   pvtime

   ptp_kvm

   vcpu-features

Documentation/virt/kvm/arm/pkvm.rst

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.. SPDX-License-Identifier: GPL-2.0



====================

Protected KVM (pKVM)

====================



**NOTE**: pKVM is currently an experimental, development feature and

subject to breaking changes as new isolation features are implemented.

Please reach out to the developers at kvmarm@lists.linux.dev if you have

any questions.



Overview

========



Booting a host kernel with '``kvm-arm.mode=protected``' enables

"Protected KVM" (pKVM). During boot, pKVM installs a stage-2 identity

map page-table for the host and uses it to isolate the hypervisor

running at EL2 from the rest of the host running at EL1/0.



pKVM permits creation of protected virtual machines (pVMs) by passing

the ``KVM_VM_TYPE_ARM_PROTECTED`` machine type identifier to the

``KVM_CREATE_VM`` ioctl(). The hypervisor isolates pVMs from the host by

unmapping pages from the stage-2 identity map as they are accessed by a

pVM. Hypercalls are provided for a pVM to share specific regions of its

IPA space back with the host, allowing for communication with the VMM.

A Linux guest must be configured with ``CONFIG_ARM_PKVM_GUEST=y`` in

order to issue these hypercalls.



See hypercalls.rst for more details.



Isolation mechanisms

====================



pKVM relies on a number of mechanisms to isolate PVMs from the host:



CPU memory isolation

--------------------



Status: Isolation of anonymous memory and metadata pages.



Metadata pages (e.g. page-table pages and '``struct kvm_vcpu``' pages)

are donated from the host to the hypervisor during pVM creation and

are consequently unmapped from the stage-2 identity map until the pVM is

destroyed.



Similarly to regular KVM, pages are lazily mapped into the guest in

response to stage-2 page faults handled by the host. However, when

running a pVM, these pages are first pinned and then unmapped from the

stage-2 identity map as part of the donation procedure. This gives rise

to some user-visible differences when compared to non-protected VMs,

largely due to the lack of MMU notifiers:



* Memslots cannot be moved or deleted once the pVM has started running.

* Read-only memslots and dirty logging are not supported.

* With the exception of swap, file-backed pages cannot be mapped into a

  pVM.

* Donated pages are accounted against ``RLIMIT_MLOCK`` and so the VMM

  must have a sufficient resource limit or be granted ``CAP_IPC_LOCK``.

  The lack of a runtime reclaim mechanism means that memory locked for

  a pVM will remain locked until the pVM is destroyed.

* Changes to the VMM address space (e.g. a ``MAP_FIXED`` mmap() over a

  mapping associated with a memslot) are not reflected in the guest and

  may lead to loss of coherency.

* Accessing pVM memory that has not been shared back will result in the

  delivery of a SIGSEGV.

* If a system call accesses pVM memory that has not been shared back

  then it will either return ``-EFAULT`` or forcefully reclaim the

  memory pages. Reclaimed memory is zeroed by the hypervisor and a

  subsequent attempt to access it in the pVM will return ``-EFAULT``

  from the ``VCPU_RUN`` ioctl().



CPU state isolation

-------------------



Status: **Unimplemented.**



DMA isolation using an IOMMU

----------------------------



Status: **Unimplemented.**



Proxying of Trustzone services

------------------------------



Status: FF-A and PSCI calls from the host are proxied by the pKVM

hypervisor.



The FF-A proxy ensures that the host cannot share pVM or hypervisor

memory with Trustzone as part of a "confused deputy" attack.



The PSCI proxy ensures that CPUs always have the stage-2 identity map

installed when they are executing in the host.



Protected VM firmware (pvmfw)

-----------------------------



Status: **Unimplemented.**



Resources

=========



Quentin Perret's KVM Forum 2022 talk entitled "Protected KVM on arm64: A

technical deep dive" remains a good resource for learning more about

pKVM, despite some of the details having changed in the meantime:



https://www.youtube.com/watch?v=9npebeVFbFw