Allow userspace to access the guest's virtual counter-timer offset
through the ONE_REG interface. The value read or written is defined to
be an offset from the guest's physical counter-timer. Add some
documentation to clarify how a VMM should use this and the existing
CNTVCT_EL0.

Signed-off-by: Oliver Upton <redacted>
---
 Documentation/virt/kvm/api.rst    | 10 ++++++++++
 arch/arm64/include/uapi/asm/kvm.h |  1 +
 arch/arm64/kvm/arch_timer.c       | 11 +++++++++++
 arch/arm64/kvm/guest.c            |  6 +++++-
 include/kvm/arm_arch_timer.h      |  1 +
 5 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 8d4a3471ad9e..28a65dc89985 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst

@@ -2487,6 +2487,16 @@ arm64 system registers have the following id bit patterns::
      derived from the register encoding for CNTV_CVAL_EL0.  As this is
      API, it must remain this way.
 
+.. warning::
+
+     The value of KVM_REG_ARM_TIMER_OFFSET is defined as an offset from
+     the guest's view of the physical counter-timer.
+
+     Userspace should use either KVM_REG_ARM_TIMER_OFFSET or
+     KVM_REG_ARM_TIMER_CVAL to pause and resume a guest's virtual

+     counter-timer. Mixed use of these registers could result in an
+     unpredictable guest counter value.
+
 arm64 firmware pseudo-registers have the following bit pattern::
 
   0x6030 0000 0014 <regno:16>

diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index b3edde68bc3e..949a31bc10f0 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h

@@ -255,6 +255,7 @@ struct kvm_arm_copy_mte_tags {
 #define KVM_REG_ARM_TIMER_CTL		ARM64_SYS_REG(3, 3, 14, 3, 1)
 #define KVM_REG_ARM_TIMER_CVAL		ARM64_SYS_REG(3, 3, 14, 0, 2)
 #define KVM_REG_ARM_TIMER_CNT		ARM64_SYS_REG(3, 3, 14, 3, 2)
+#define KVM_REG_ARM_TIMER_OFFSET	ARM64_SYS_REG(3, 4, 14, 0, 3)