[PATCH v6 13/21] KVM: arm64: Allow userspace to configure a vCPU's virtual offset

[Oliver Upton <oupton@google.com>]

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 <oupton@google.com>
---
 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)
 
 /* KVM-as-firmware specific pseudo-registers */
 #define KVM_REG_ARM_FW			(0x0014 << KVM_REG_ARM_COPROC_SHIFT)
diff --git a/arch/arm64/kvm/arch_timer.c b/arch/arm64/kvm/arch_timer.c
index 4c2b763a8849..a8815b09da3e 100644
--- a/arch/arm64/kvm/arch_timer.c
+++ b/arch/arm64/kvm/arch_timer.c
@@ -868,6 +868,10 @@ int kvm_arm_timer_set_reg(struct kvm_vcpu *vcpu, u64 regid, u64 value)
 		timer = vcpu_vtimer(vcpu);
 		kvm_arm_timer_write(vcpu, timer, TIMER_REG_CVAL, value);
 		break;
+	case KVM_REG_ARM_TIMER_OFFSET:
+		timer = vcpu_vtimer(vcpu);
+		update_vtimer_cntvoff(vcpu, value);
+		break;
 	case KVM_REG_ARM_PTIMER_CTL:
 		timer = vcpu_ptimer(vcpu);
 		kvm_arm_timer_write(vcpu, timer, TIMER_REG_CTL, value);
@@ -912,6 +916,9 @@ u64 kvm_arm_timer_get_reg(struct kvm_vcpu *vcpu, u64 regid)
 	case KVM_REG_ARM_TIMER_CVAL:
 		return kvm_arm_timer_read(vcpu,
 					  vcpu_vtimer(vcpu), TIMER_REG_CVAL);
+	case KVM_REG_ARM_TIMER_OFFSET:
+		return kvm_arm_timer_read(vcpu,
+					  vcpu_vtimer(vcpu), TIMER_REG_OFFSET);
 	case KVM_REG_ARM_PTIMER_CTL:
 		return kvm_arm_timer_read(vcpu,
 					  vcpu_ptimer(vcpu), TIMER_REG_CTL);
@@ -949,6 +956,10 @@ static u64 kvm_arm_timer_read(struct kvm_vcpu *vcpu,
 		val = kvm_phys_timer_read() - timer_get_offset(timer);
 		break;
 
+	case TIMER_REG_OFFSET:
+		val = timer_get_offset(timer);
+		break;
+
 	default:
 		BUG();
 	}
diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
index 1dfb83578277..17fc06e2b422 100644
--- a/arch/arm64/kvm/guest.c
+++ b/arch/arm64/kvm/guest.c
@@ -591,7 +591,7 @@ static unsigned long num_core_regs(const struct kvm_vcpu *vcpu)
  * ARM64 versions of the TIMER registers, always available on arm64
  */
 
-#define NUM_TIMER_REGS 3
+#define NUM_TIMER_REGS 4
 
 static bool is_timer_reg(u64 index)
 {
@@ -599,6 +599,7 @@ static bool is_timer_reg(u64 index)
 	case KVM_REG_ARM_TIMER_CTL:
 	case KVM_REG_ARM_TIMER_CNT:
 	case KVM_REG_ARM_TIMER_CVAL:
+	case KVM_REG_ARM_TIMER_OFFSET:
 		return true;
 	}
 	return false;
@@ -614,6 +615,9 @@ static int copy_timer_indices(struct kvm_vcpu *vcpu, u64 __user *uindices)
 	uindices++;
 	if (put_user(KVM_REG_ARM_TIMER_CVAL, uindices))
 		return -EFAULT;
+	uindices++;
+	if (put_user(KVM_REG_ARM_TIMER_OFFSET, uindices))
+		return -EFAULT;
 
 	return 0;
 }
diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
index 9d65d4a29f81..615f9314f6a5 100644
--- a/include/kvm/arm_arch_timer.h
+++ b/include/kvm/arm_arch_timer.h
@@ -21,6 +21,7 @@ enum kvm_arch_timer_regs {
 	TIMER_REG_CVAL,
 	TIMER_REG_TVAL,
 	TIMER_REG_CTL,
+	TIMER_REG_OFFSET,
 };
 
 struct arch_timer_context {
-- 
2.32.0.605.g8dce9f2422-goog