Re: [RFC 00/55] Nested Virtualization on KVM/ARM

From: Christoffer Dall
Date: Wed Feb 22 2017 - 13:24:04 EST

Hi Jintack,

On Mon, Jan 09, 2017 at 01:23:56AM -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).
> This series supports nested virtualization on arm64. ARM recently announced an
> extension (ARMv8.3) which has support for nested virtualization[1]. This series
> is based on the ARMv8.3 specification.
> Supporting nested virtualization means that the hypervisor provides not only
> EL0/EL1 execution environment with VMs as it usually does, but also the
> virtualization extensions including EL2 execution environment with the VMs.
> Once the host hypervisor provides those execution environment with the VMs,
> then the guest hypervisor can run its own VMs (nested VMs) naturally.
> To support nested virtualization on ARM the hypervisor must emulate a virtual
> execution environment consisting of EL2, EL1, and EL0, as the guest hypervisor
> will run in a virtual EL2 mode. Normally KVM/ARM only emulated a VM supporting
> EL1/0 running in their respective native CPU modes, but with nested
> virtualization we deprivilege the guest hypervisor and emulate a virtual EL2
> execution mode in EL1 using the hardware features provided by ARMv8.3 to trap
> EL2 operations to EL1. To do that the host hypervisor needs to manage EL2
> register state for the guest hypervisor, and shadow EL1 register state that
> reflects the EL2 register state to run the guest hypervisor in EL1. See patch 6
> through 10 for this.
> For memory virtualization, the biggest issue is that we now have more than two
> stages of translation when running nested VMs. We choose to merge two stage-2
> page tables (one from the guest hypervisor and the other from the host
> hypervisor) and create shadow stage-2 page tables, which have mappings from the
> nested VMâs physical addresses to the machine physical addresses. Stage-1
> translation is done by the hardware as is done for the normal VMs.
> To provide VGIC support to the guest hypervisor, we emulate the GIC
> virtualization extensions using trap-and-emulate to a virtual GIC Hypervisor
> Control Interface. Furthermore, we can still use the GIC VE hardware features
> to deliver virtual interrupts to the nested VM, by directly mapping the GIC
> VCPU interface to the nested VM and switching the content of the GIC Hypervisor
> Control interface when alternating between a nested VM and a normal VM. See
> patches 25 through 32, and 50 through 52 for more information.
> For timer virtualization, the guest hypervisor expects to have access to the
> EL2 physical timer, the EL1 physical timer and the virtual timer. So, the host
> hypervisor needs to provide all of them. The virtual timer is always available
> to VMs. The physical timer is available to VMs via my previous patch series[3].
> The EL2 physical timer is not supported yet in this RFC. We plan to support
> this as it is required to run other guest hypervisors such as Xen.
> Even though this work is not complete (see limitations below), I'd appreciate
> early feedback on this RFC. Specifically, I'm interested in:
> - Is it better to have a kernel config or to make it configurable at runtime?
> - I wonder if the data structure for memory management makes sense.
> - What architecture version do we support for the guest hypervisor, and how?
> For example, do we always support all architecture versions or the same
> architecture as the underlying hardware platform? Or is it better
> to make it configurable from the userspace?
> - Initial comments on the overall design?
> This patch series is based on kvm-arm-for-4.9-rc7 with the patch series to provide
> VMs with the EL1 physical timer[2].
> Git:
> Testing:
> We have tested this on ARMv8.0 (Applied Micro X-Gene)[3] since ARMv8.3 hardware
> is not available yet. We have paravirtualized the guest hypervisor to trap to
> EL2 as specified in ARMv8.3 specification using hvc instruction. We plan to
> test this on ARMv8.3 model, and will post the result and v2 if necessary.
> Limitations:
> - This patch series only supports arm64, not arm. All the patches compile on
> arm, but I haven't try to boot normal VMs on it.
> - The guest hypervisor with VHE (ARMv8.1) is not supported in this RFC. I have
> patches for that, but they need to be cleaned up.
> - Recursive nesting (i.e. emulating ARMv8.3 in the VM) is not tested yet.
> - Other hypervisors (such as Xen) on KVM are not tested.
> - Test to boot normal VMs on arm architecture
> - Test this on ARMv8.3 model
> - Support the guest hypervisor with VHE
> - Provide the guest hypervisor with the EL2 physical timer
> - Run other hypervisors such as Xen on KVM

I have a couple of overall questions and comments on this series:

First, I think we should make sure that the series actually works with
v8.3 on the model using both VHE and non-VHE for the host hypervisor.

Second, this patch set is pretty large overall and it would be great if
we could split it up into some slightly more manageable bits. I'm not
exactly how to do that, but perhaps we can rework it so that we add bits
of framework (CPU, memory, interrupt, timers) as individual series, and
finally we plug all the logic together with the current flow. What do
you think?

Third, we should follow the feedback from David about not using a kernel
config option. I'm afraid that some code will bitrot too fast if guided
by a kernel config option, so a runtime parameter and using static keys
where relevant seems like a better approach to me. But since KVM/ARM is
not loaded as a module, this would have to be a kernel cmdline
parameter. What do people think?

Fourth, there are some places where we have hard-coded information (like
the location of the GICH/GICV interfaces) which have to be fixed by
adding the required userspace interfaces.

Fifth, the ordering of the patches needs a bit of love. I think it's
important that we build the whole infrastructure first, but leave it
completely disabled until the end, and then we plug in all the
capabilities of userspace to create a nested VM in the end. So for
example, I would expect that patch 03 would be the last patch in the

Overall though, this is a massive amount of work, and it's awesome that
you were able to pull it together to a pretty nice initial RFC!