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: 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>

Documentation/admin-guide/kernel-parameters.txt

@@ -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

@@ -10,6 +10,7 @@ ARM
    fw-pseudo-registers
    hyp-abi
    hypercalls
+   pkvm
    pvtime
    ptp_kvm
    vcpu-features

Documentation/virt/kvm/arm/pkvm.rst

@@ -0,0 +1,106 @@
+.. 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