[RFC PATCH v2 00/38] Nested Virtualization on KVM/ARM

[Jintack Lim]

Nested virtualization is the ability to run a virtual machine inside another
virtual machine. In other words, itâs about running a hypervisor (the guest
hypervisor) on top of another hypervisor (the host hypervisor).

Supporting nested virtualization on ARM means that the hypervisor provides not
only EL0/EL1 execution environment to VMs as it usually does but also the
virtualization extensions including EL2 execution environment. Once the host
hypervisor provides those execution environments to the VMs, then the guest
hypervisor can run its own VMs (nested VMs) naturally.

This series supports nested virtualization on arm64. ARM recently announced an
extension (ARMv8.3) which has support for nested virtualization[1]. This patch
set is based on the ARMv8.3 specification and tested on the FastModel with
ARMv8.3 extension.

The whole patch set to support nested virtualization is huge over 70
patches, so I categorized them into four parts: CPU, memory, VGIC, and timer
virtualization. This patch series is the first part.
 
CPU virtualization patch series provides basic nested virtualization framework
and instruction emulations including v8.1 VHE feature and v8.3 nested
virtualization feature for VMs.

This patch series again can be divided into four parts. Patch 1 to 5 introduces
nested virtualization by discovering hardware feature, adding a kernel
parameter and allowing the userspace to set the initial CPU mode to EL2.

Patch 6 to 25 are to support the EL2 execution environment, the virtual EL2, to
a VM on v8.0 architecture. We de-privilege the guest hypervisor and emulate the
virtual EL2 mode in EL1 using the hardware features provided by ARMv8.3; The
host hypervisor manages virtual EL2 register state for the guest hypervisor
and shadow EL1 register state that reflects the virtual EL2 register state to
run the guest hypervisor in EL1. 

Patch 26 to 33 add support for the virtual EL2 with Virtualization Host
Extensions. These patches emulate newly defined registers and bits in v8.1 and
allow the virtual EL2 to access EL2 register states via EL1 register accesses
as in the real EL2.

Patch 34 to 38 are to support for the virtual EL2 with nested virtualization.
These enable recursive nested virtualization.

This patch set is tested on the FastModel with the v8.3 extension for arm64 and
a cubietruck for arm32. On the FastModel, the host and the guest kernels are
compiled with and without VHE, so there are four combinations. I was able to
boot SMP Linux in the nested VM on all four configurations and able to run
hackbench. I also checked that regular VMs could boot when the nested
virtualization kernel parameter was not set. On the cubietruck, I also verified
that regular VMs could boot as well.

I'll share my experiment setup shortly.

Even though this work has some limitations and TODOs, I'd appreciate early
feedback on this RFC. Specifically, I'm interested in:

- Overall design to manage vcpu context for the virtual EL2
- Verifying correct EL2 register configurations such as HCR_EL2, CPTR_EL2
  (Patch 30 and 32)
- Patch organization and coding style

This patch series is based on kvm/next d38338e.
The whole patch series including memory, VGIC, and timer patches is available
here:

git@xxxxxxxxxx:columbia/nesting-pub.git rfc-v2

Limitations:
- There are some cases that the target exception level of a VM is ambiguous when
  emulating eret instruction. I'm discussing this issue with Christoffer and
  Marc. Meanwhile, I added a temporary patch (not included in this
  series. f1beaba in the repo) and used 4.10.0 kernel when testing the guest
  hypervisor with VHE.
- Recursive nested virtualization is not tested yet.
- Other hypervisors (such as Xen) on KVM are not tested.

TODO:
- Submit memory, VGIC, and timer patches
- Evaluate regular VM performance to see if there's a negative impact.
- Test other hypervisors such as Xen on KVM
- Test recursive nested virtualization

v1-->v2:
- Added support for the virtual EL2 with VHE
- Rewrote commit messages and comments from the perspective of supporting
  execution environments to VMs, rather than from the perspective of the guest
  hypervisor running in them.
- Fixed a few bugs to make it run on the FastModel.
- Tested on ARMv8.3 with four configurations. (host/guest. with/without VHE.)
- Rebased to kvm/next

[1] https://www.community.arm.com/processors/b/blog/posts/armv8-a-architecture-2016-additions

Christoffer Dall (7):
  KVM: arm64: Add KVM nesting feature
  KVM: arm64: Allow userspace to set PSR_MODE_EL2x
  KVM: arm64: Add vcpu_mode_el2 primitive to support nesting
  KVM: arm/arm64: Add a framework to prepare virtual EL2 execution
  arm64: Add missing TCR hw defines
  KVM: arm64: Create shadow EL1 registers
  KVM: arm64: Trap EL1 VM register accesses in virtual EL2

Jintack Lim (31):
  arm64: Add ARM64_HAS_NESTED_VIRT feature
  KVM: arm/arm64: Enable nested virtualization via command-line
  KVM: arm/arm64: Check if nested virtualization is in use
  KVM: arm64: Add EL2 system registers to vcpu context
  KVM: arm64: Add EL2 special registers to vcpu context
  KVM: arm64: Add the shadow context for virtual EL2 execution
  KVM: arm64: Set vcpu context depending on the guest exception level
  KVM: arm64: Synchronize EL1 system registers on virtual EL2 entry and
    exit
  KVM: arm64: Move exception macros and enums to a common file
  KVM: arm64: Support to inject exceptions to the virtual EL2
  KVM: arm64: Trap SPSR_EL1, ELR_EL1 and VBAR_EL1 from virtual EL2
  KVM: arm64: Trap CPACR_EL1 access in virtual EL2
  KVM: arm64: Handle eret instruction traps
  KVM: arm64: Set a handler for the system instruction traps
  KVM: arm64: Handle PSCI call via smc from the guest
  KVM: arm64: Inject HVC exceptions to the virtual EL2
  KVM: arm64: Respect virtual HCR_EL2.TWX setting
  KVM: arm64: Respect virtual CPTR_EL2.TFP setting
  KVM: arm64: Add macros to support the virtual EL2 with VHE
  KVM: arm64: Add EL2 registers defined in ARMv8.1 to vcpu context
  KVM: arm64: Emulate EL12 register accesses from the virtual EL2
  KVM: arm64: Support a VM with VHE considering EL0 of the VHE host
  KVM: arm64: Allow the virtual EL2 to access EL2 states without trap
  KVM: arm64: Manage the shadow states when virtual E2H bit enabled
  KVM: arm64: Trap and emulate CPTR_EL2 accesses via CPACR_EL1 from the
    virtual EL2 with VHE
  KVM: arm64: Emulate appropriate VM control system registers
  KVM: arm64: Respect the virtual HCR_EL2.NV bit setting
  KVM: arm64: Respect the virtual HCR_EL2.NV bit setting for EL12
    register traps
  KVM: arm64: Respect virtual HCR_EL2.TVM and TRVM settings
  KVM: arm64: Respect the virtual HCR_EL2.NV1 bit setting
  KVM: arm64: Respect the virtual CPTR_EL2.TCPAC setting

 Documentation/admin-guide/kernel-parameters.txt |   4 +
 arch/arm/include/asm/kvm_emulate.h              |  17 ++
 arch/arm/include/asm/kvm_host.h                 |  15 +
 arch/arm64/include/asm/cpucaps.h                |   3 +-
 arch/arm64/include/asm/esr.h                    |   1 +
 arch/arm64/include/asm/kvm_arm.h                |   2 +
 arch/arm64/include/asm/kvm_coproc.h             |   3 +-
 arch/arm64/include/asm/kvm_emulate.h            |  56 ++++
 arch/arm64/include/asm/kvm_host.h               |  64 ++++-
 arch/arm64/include/asm/kvm_hyp.h                |  24 --
 arch/arm64/include/asm/pgtable-hwdef.h          |   6 +
 arch/arm64/include/asm/sysreg.h                 |  70 +++++
 arch/arm64/include/uapi/asm/kvm.h               |   1 +
 arch/arm64/kernel/asm-offsets.c                 |   1 +
 arch/arm64/kernel/cpufeature.c                  |  11 +
 arch/arm64/kvm/Makefile                         |   5 +-
 arch/arm64/kvm/context.c                        | 346 +++++++++++++++++++++++
 arch/arm64/kvm/emulate-nested.c                 |  83 ++++++
 arch/arm64/kvm/guest.c                          |   2 +
 arch/arm64/kvm/handle_exit.c                    |  89 +++++-
 arch/arm64/kvm/hyp/entry.S                      |  13 +
 arch/arm64/kvm/hyp/hyp-entry.S                  |   2 +-
 arch/arm64/kvm/hyp/switch.c                     |  33 ++-
 arch/arm64/kvm/hyp/sysreg-sr.c                  | 117 ++++----
 arch/arm64/kvm/inject_fault.c                   |  12 -
 arch/arm64/kvm/nested.c                         |  63 +++++
 arch/arm64/kvm/reset.c                          |   8 +
 arch/arm64/kvm/sys_regs.c                       | 359 +++++++++++++++++++++++-
 arch/arm64/kvm/sys_regs.h                       |   8 +
 arch/arm64/kvm/trace.h                          |  43 ++-
 virt/kvm/arm/arm.c                              |  20 ++
 31 files changed, 1363 insertions(+), 118 deletions(-)
 create mode 100644 arch/arm64/kvm/context.c
 create mode 100644 arch/arm64/kvm/emulate-nested.c
 create mode 100644 arch/arm64/kvm/nested.c

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1.9.1