Re: [PATCH v4 5/5] x86, kvm: support vcpu preempted check

This is to fix some lock holder preemption issues. Some other locks
implementation do a spin loop before acquiring the lock itself.
Currently kernel has an interface of bool vcpu_is_preempted(int cpu). It
takes the cpu as parameter and return true if the cpu is preempted.  Then
kernel can break the spin loops upon on the retval of vcpu_is_preempted.

As kernel has used this interface, So lets support it.

We use one field of struct kvm_steal_time to indicate that if one vcpu
is running or not.

unix benchmark result:
host:  kernel 4.8.1, i5-4570, 4 cpus
guest: kernel 4.8.1, 8 vcpus

	test-case			after-patch	  before-patch
Execl Throughput                       |    18307.9 lps  |    11701.6 lps 
File Copy 1024 bufsize 2000 maxblocks  |  1352407.3 KBps |   790418.9 KBps
File Copy 256 bufsize 500 maxblocks    |   367555.6 KBps |   222867.7 KBps
File Copy 4096 bufsize 8000 maxblocks  |  3675649.7 KBps |  1780614.4 KBps
Pipe Throughput                        | 11872208.7 lps  | 11855628.9 lps 
Pipe-based Context Switching           |  1495126.5 lps  |  1490533.9 lps 
Process Creation                       |    29881.2 lps  |    28572.8 lps 
Shell Scripts (1 concurrent)           |    23224.3 lpm  |    22607.4 lpm 
Shell Scripts (8 concurrent)           |     3531.4 lpm  |     3211.9 lpm 
System Call Overhead                   | 10385653.0 lps  | 10419979.0 lps 

Signed-off-by: Pan Xinhui <redacted>
---

diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/paravirt_types.h

@@ -98,6 +98,10 @@ struct pv_time_ops {
 	unsigned long long (*steal_clock)(int cpu);
 };
 
+struct pv_vcpu_ops {
+	bool (*vcpu_is_preempted)(int cpu);
+};
+

diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h

@@ -45,7 +45,8 @@ struct kvm_steal_time {
 	__u64 steal;
 	__u32 version;
 	__u32 flags;
-	__u32 pad[12];
+	__u32 preempted;

diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c

@@ -415,6 +415,15 @@ void kvm_disable_steal_time(void)
+static bool kvm_vcpu_is_preempted(int cpu)
+{
+	struct kvm_steal_time *src;
+
+	src = &per_cpu(steal_time, cpu);
+
+	return !!src->preempted;
+}
+
 #ifdef CONFIG_SMP
 static void __init kvm_smp_prepare_boot_cpu(void)
 {

@@ -488,6 +497,8 @@ void __init kvm_guest_init(void)
 	kvm_guest_cpu_init();
 #endif
 
+	pv_vcpu_ops.vcpu_is_preempted = kvm_vcpu_is_preempted;

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c

@@ -2057,6 +2057,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time))))
 		return;
 
+	vcpu->arch.st.steal.preempted = 0;
+
 	if (vcpu->arch.st.steal.version & 1)
 		vcpu->arch.st.steal.version += 1;  /* first time write, random junk */
 

@@ -2812,6 +2814,16 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 {
+	if (vcpu->arch.st.msr_val & KVM_MSR_ENABLED)
+		if (kvm_read_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
+					&vcpu->arch.st.steal,
+					sizeof(struct kvm_steal_time)) == 0) {
+			vcpu->arch.st.steal.preempted = 1;
+			kvm_write_guest_cached(vcpu->kvm, &vcpu->arch.st.stime,
+					&vcpu->arch.st.steal,
+					sizeof(struct kvm_steal_time));
+		}