[PATCH 2/4] arm64/x86: KVM: Introduce steal time cap

[Andrew Jones <drjones@redhat.com>]

arm64 requires a vcpu fd (KVM_HAS_DEVICE_ATTR vcpu ioctl) to probe
support for steal time. However this is unnecessary and complicates
userspace (userspace may prefer delaying vcpu creation until after
feature probing). Since probing steal time only requires a KVM fd,
we introduce a cap that can be checked. Additionally, when probing
steal time we should check delayacct_on, because even though
CONFIG_KVM selects TASK_DELAY_ACCT, it's possible for the host
kernel to have delay accounting disabled with the 'nodelayacct'
command line option. x86 already determines support for steal time
by checking delayacct_on and can already probe steal time support
with a kvm fd (KVM_GET_SUPPORTED_CPUID), but we add the cap there
too for consistency.

Signed-off-by: Andrew Jones <drjones@redhat.com>
---
 Documentation/virt/kvm/api.rst | 11 +++++++++++
 arch/arm64/kvm/arm.c           |  3 +++
 arch/x86/kvm/x86.c             |  3 +++
 include/uapi/linux/kvm.h       |  1 +
 4 files changed, 18 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 9a12ea498dbb..05b1fdb88383 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6151,3 +6151,14 @@ KVM can therefore start protected VMs.
 This capability governs the KVM_S390_PV_COMMAND ioctl and the
 KVM_MP_STATE_LOAD MP_STATE. KVM_SET_MP_STATE can fail for protected
 guests when the state change is invalid.
+
+8.24 KVM_CAP_STEAL_TIME
+-----------------------
+
+:Architectures: arm64, x86
+
+This capability indicates that KVM supports steal time accounting.
+When steal time accounting is supported it may be enabled with
+architecture-specific interfaces.  For x86 see
+Documentation/virt/kvm/msr.rst "MSR_KVM_STEAL_TIME".  For arm64 see
+Documentation/virt/kvm/devices/vcpu.rst "KVM_ARM_VCPU_PVTIME_CTRL"
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 90cb90561446..f6dca6d09952 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -222,6 +222,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		 */
 		r = 1;
 		break;
+	case KVM_CAP_STEAL_TIME:
+		r = sched_info_on();
+		break;
 	default:
 		r = kvm_arch_vm_ioctl_check_extension(kvm, ext);
 		break;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 00c88c2f34e4..ced6335e403e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3533,6 +3533,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_HYPERV_ENLIGHTENED_VMCS:
 		r = kvm_x86_ops.nested_ops->enable_evmcs != NULL;
 		break;
+	case KVM_CAP_STEAL_TIME:
+		r = sched_info_on();
+		break;
 	default:
 		break;
 	}
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 4fdf30316582..121fb29ac004 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1031,6 +1031,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_PPC_SECURE_GUEST 181
 #define KVM_CAP_HALT_POLL 182
 #define KVM_CAP_ASYNC_PF_INT 183
+#define KVM_CAP_STEAL_TIME 184
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.25.4