[RFC v2 00/10] Provide the EL1 physical timer to the VM

From: Jintack Lim
Date: Thu Jan 26 2017 - 20:06:52 EST

The ARM architecture defines the EL1 physical timer and the virtual timer,
and it is reasonable for an OS to expect to be able to access both.
However, the current KVM implementation does not provide the EL1 physical
timer to VMs but terminates VMs on access to the timer.

This patch series enables VMs to use the EL1 physical timer through
trap-and-emulate. The KVM host emulates each EL1 physical timer register
access and sets up the background timer accordingly. When the background
timer expires, the KVM host injects EL1 physical timer interrupts to the
VM. Alternatively, it's also possible to allow VMs to access the EL1
physical timer without trapping. However, this requires somehow using the
EL2 physical timer for the Linux host while running the VM instead of the
EL1 physical timer. Right now I just implemented trap-and-emulate because
this was straightforward to do, and I leave it to future work to determine
if transferring the EL1 physical timer state to the EL2 timer provides any
performance benefit.

This feature will be useful for any OS that wishes to access the EL1
physical timer. Nested virtualization is one of those use cases. A nested
hypervisor running inside a VM would think it has full access to the
hardware and naturally tries to use the EL1 physical timer as Linux would
do. Other nested hypervisors may try to use the EL2 physical timer as Xen
would do, but supporting the EL2 physical timer to the VM is out of scope
of this patch series. This patch series will make it easy to add the EL2
timer support in the future, though.

Note that Linux VMs booting in EL1 will be unaffected by this patch series
and will continue to use only the virtual timer and this patch series will
therefore not introduce any performance degredation as a result of

v1 => v2:
- Rebase on kvm-arm-for-4.10-rc4
- To make it simple, schedule the background timer for the EL1 physical timer
emulation on every entry to the VM and cancel it on exit.
- Change timer_context structure to have cntvoff and restore enable field back
to arch_timer_cpu structure

Jintack Lim (10):
KVM: arm/arm64: Abstract virtual timer context into separate structure
KVM: arm/arm64: Move cntvoff to each timer context
KVM: arm/arm64: Decouple kvm timer functions from virtual timer
KVM: arm/arm64: Add the EL1 physical timer context
KVM: arm/arm64: Initialize the emulated EL1 physical timer
KVM: arm/arm64: Update the physical timer interrupt level
KVM: arm/arm64: Set a background timer to the earliest timer
KVM: arm/arm64: Set up a background timer for the physical timer
KVM: arm64: Add the EL1 physical timer access handler
KVM: arm/arm64: Emulate the EL1 phys timer register access

arch/arm/include/asm/kvm_host.h | 6 +-
arch/arm/kvm/arm.c | 3 +-
arch/arm/kvm/reset.c | 9 +-
arch/arm64/include/asm/kvm_host.h | 4 +-
arch/arm64/kvm/reset.c | 9 +-
arch/arm64/kvm/sys_regs.c | 60 +++++++++++
include/kvm/arm_arch_timer.h | 39 ++++----
virt/kvm/arm/arch_timer.c | 204 ++++++++++++++++++++++++++++----------
virt/kvm/arm/hyp/timer-sr.c | 13 +--
9 files changed, 261 insertions(+), 86 deletions(-)