[PATCH resend] x86,kvm: Add a kernel parameter to disable PV spinlock

[Oscar Salvador]

This is just a resend of Waiman Long's patch.
I could not find why it was not merged to upstream, so I thought
to give it another chance.
What follows is what Waiman Long wrote.

Xen has an kernel command line argument "xen_nopvspin" to disable
paravirtual spinlocks. This patch adds a similar "kvm_nopvspin"
argument to disable paravirtual spinlocks for KVM. This can be useful
for testing as well as allowing administrators to choose unfair lock
for their KVM guests if they want to.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  6 +++++-
 arch/x86/kernel/kvm.c                           | 14 +++++++++++++-
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index d9c171ce4190..56c6e3acdf8e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1899,6 +1899,10 @@
 			feature (tagged TLBs) on capable Intel chips.
 			Default is 1 (enabled)
 
+	kvm_nopvspin	[X86,KVM]
+			Disables the paravirtualized spinlock slowpath
+			optimizations for KVM.
+
 	l2cr=		[PPC]
 
 	l3cr=		[PPC]
@@ -4533,7 +4537,7 @@
 			never -- do not unplug even if version check succeeds
 
 	xen_nopvspin	[X86,XEN]
-			Disables the ticketlock slowpath using Xen PV
+			Disables the spinlock slowpath using Xen PV
 			optimizations.
 
 	xen_nopv	[X86]
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index d04e30e3c0ff..51addf874fc1 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -568,6 +568,18 @@ static void kvm_kick_cpu(int cpu)
 	kvm_hypercall2(KVM_HC_KICK_CPU, flags, apicid);
 }
 
+static bool kvm_pvspin = true;
+
+/*
+ * Allow disabling of PV spinlock in kernel command line
+ */
+static __init int kvm_parse_nopvspin(char *arg)
+{
+	kvm_pvspin = false;
+	return 0;
+}
+early_param("kvm_nopvspin", kvm_parse_nopvspin);
+
 #include <asm/qspinlock.h>
 
 static void kvm_wait(u8 *ptr, u8 val)
@@ -633,7 +645,7 @@ asm(
  */
 void __init kvm_spinlock_init(void)
 {
-	if (!kvm_para_available())
+	if (!kvm_para_available() || !kvm_pvspin)
 		return;
 	/* Does host kernel support KVM_FEATURE_PV_UNHALT? */
 	if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT))
-- 
2.13.5