[PATCH v4 05/11] KVM: x86/xen: Consolidate checks on Xen vCPU ID for singleshot timer hypercalls

[Sean Christopherson <seanjc@google.com>]

Hoist the checks on the Xen vCPU ID when handling set_singleshot_timer and
stop_singleshot_timer hypercalls out of their individual if-statements,
so that both checks on the ID are in common code.  kvm_xen_hcall_vcpu_op()
is already doubly committed to handling only singleshot timer hypercalls,
and even if that were to change in the future, the function could simply
be renamed and turned into a helper specifically for timer hypercalls.

Opportunistically add a comment to explain why the check exists; the code
looks rather nonsensical without the knowledge that @vcpu_id is a common
param for all per-vCPU hypercalls.

No functional change intended.

Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/xen.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 3ed6686e0a1a..7a0d89faca85 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1614,12 +1614,18 @@ static bool kvm_xen_hcall_vcpu_op(struct kvm_vcpu *vcpu, bool longmode, int cmd,
 	if (!kvm_xen_timer_enabled(vcpu))
 		return false;
 
+	/*
+	 * Reject the hypercall if the guest is trying to start/stop the timer
+	 * for a different vCPU.  Xen per-vCPU hypercalls take a target vCPU as
+	 * a common parameter, as all per-vCPU hypercalls *except* single-shot
+	 * timer updates can be cross-vCPU.
+	 */
+	if (vcpu->arch.xen.vcpu_id != vcpu_id) {
+		*r = -EINVAL;
+		return true;
+	}
+
 	if (cmd == VCPUOP_set_singleshot_timer) {
-		if (vcpu->arch.xen.vcpu_id != vcpu_id) {
-			*r = -EINVAL;
-			return true;
-		}
-
 		/*
 		 * The only difference for 32-bit compat is the 4 bytes of
 		 * padding after the interesting part of the structure. So
@@ -1644,10 +1650,6 @@ static bool kvm_xen_hcall_vcpu_op(struct kvm_vcpu *vcpu, bool longmode, int cmd,
 
 		kvm_xen_start_timer(vcpu, oneshot.timeout_abs_ns, false);
 	} else {
-		if (vcpu->arch.xen.vcpu_id != vcpu_id) {
-			*r = -EINVAL;
-			return true;
-		}
 		kvm_xen_stop_timer(vcpu);
 	}
 
-- 
2.55.0.rc0.799.gd6f94ed593-goog