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

[Christoffer Dall]

Hi Jintack,

On Tue, Jul 18, 2017 at 11:58:26AM -0500, Jintack Lim wrote:
> 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
> 

I think this overall looks pretty good, and I think you can drop the RFC
tag from the next revision, assuming the remaining patch sets for
memory, vgic, and timers don't require some major controversial rework
of these patches.

Thanks,
-Christoffer

> 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
> 
> -- 
> 1.9.1
>