[PATCH v4 11/13] KVM: arm64: Provide userspace access to the physical counter offset

[Andrew Jones]

On Thu, Jul 29, 2021 at 12:10:10AM +0000, Oliver Upton wrote:
> Presently, KVM provides no facilities for correctly migrating a guest
> that depends on the physical counter-timer. While most guests (barring
> NV, of course) should not depend on the physical counter-timer, an
> operator may still wish to provide a consistent view of the physical
> counter-timer across migrations.
> 
> Provide userspace with a new vCPU attribute to modify the guest physical
> counter-timer offset. Since the base architecture doesn't provide a
> physical counter-timer offset register, emulate the correct behavior by
> trapping accesses to the physical counter-timer whenever the offset
> value is non-zero.
> 
> Uphold the same behavior as CNTVOFF_EL2 and broadcast the physical
> offset to all vCPUs whenever written. This guarantees that the
> counter-timer we provide the guest remains architectural, wherein all
> views of the counter-timer are consistent across vCPUs. Reconfigure
> timer traps for VHE on every guest entry, as different VMs will now have
> different traps enabled. Enable physical counter traps for nVHE whenever
> the offset is nonzero (we already trap physical timer registers in
> nVHE).
> 
> FEAT_ECV provides a guest physical counter-timer offset register
> (CNTPOFF_EL2), but ECV-enabled hardware is nonexistent at the time of
> writing so support for it was elided for the sake of the author :)
> 
> Cc: Andrew Jones <drjones at redhat.com>
> Signed-off-by: Oliver Upton <oupton at google.com>
> ---
>  Documentation/virt/kvm/devices/vcpu.rst   | 22 +++++++
>  arch/arm64/include/asm/kvm_host.h         |  1 +
>  arch/arm64/include/asm/kvm_hyp.h          |  2 -
>  arch/arm64/include/asm/sysreg.h           |  1 +
>  arch/arm64/include/uapi/asm/kvm.h         |  1 +
>  arch/arm64/kvm/arch_timer.c               | 72 ++++++++++++++---------
>  arch/arm64/kvm/arm.c                      |  4 +-
>  arch/arm64/kvm/hyp/include/hyp/switch.h   | 23 ++++++++
>  arch/arm64/kvm/hyp/include/hyp/timer-sr.h | 26 ++++++++
>  arch/arm64/kvm/hyp/nvhe/switch.c          |  2 -
>  arch/arm64/kvm/hyp/nvhe/timer-sr.c        | 21 +++----
>  arch/arm64/kvm/hyp/vhe/timer-sr.c         | 27 +++++++++
>  include/kvm/arm_arch_timer.h              |  2 -
>  13 files changed, 158 insertions(+), 46 deletions(-)
>  create mode 100644 arch/arm64/kvm/hyp/include/hyp/timer-sr.h
>

 
Reviewed-by: Andrew Jones <drjones at redhat.com>