[PATCH] kvm: Simplify kvm_get_running_vcpu()

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

this_cpu_read() is exactly the same behavior as "preempt_disable() +
__this_cpu_read() + preempt_enable()", and it uses less instructions
in X86.

Just use this_cpu_read() to simplify kvm_get_running_vcpu().

Cc: Marc Zyngier <maz@kernel.org>
Cc: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/kvm/20200207163410.31276-1-maz@kernel.org/
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
---
 virt/kvm/kvm_main.c | 11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 14841acb8b95..0d18e9b1017f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -6306,21 +6306,14 @@ static void kvm_sched_out(struct preempt_notifier *pn,
 /**
  * kvm_get_running_vcpu - get the vcpu running on the current CPU.
  *
- * We can disable preemption locally around accessing the per-CPU variable,
- * and use the resolved vcpu pointer after enabling preemption again,
+ * The result is either NULL or the vcpu running on the current thread
  * because even if the current thread is migrated to another CPU, reading
  * the per-CPU value later will give us the same value as we update the
  * per-CPU variable in the preempt notifier handlers.
  */
 struct kvm_vcpu *kvm_get_running_vcpu(void)
 {
-	struct kvm_vcpu *vcpu;
-
-	preempt_disable();
-	vcpu = __this_cpu_read(kvm_running_vcpu);
-	preempt_enable();
-
-	return vcpu;
+	return this_cpu_read(kvm_running_vcpu);
 }
 EXPORT_SYMBOL_GPL(kvm_get_running_vcpu);
 
-- 
2.19.1.6.gb485710b