KVM: VMX: Add support for Pause-Loop Exiting

New NHM processors will support Pause-Loop Exiting by adding 2 VM-execution
control fields:
PLE_Gap    - upper bound on the amount of time between two successive
             executions of PAUSE in a loop.
PLE_Window - upper bound on the amount of time a guest is allowed to execute in
             a PAUSE loop

If the time, between this execution of PAUSE and previous one, exceeds the
PLE_Gap, processor consider this PAUSE belongs to a new loop.
Otherwise, processor determins the the total execution time of this loop(since
1st PAUSE in this loop), and triggers a VM exit if total time exceeds the
PLE_Window.
* Refer SDM volume 3b section 21.6.13 & 22.1.3.

Pause-Loop Exiting can be used to detect Lock-Holder Preemption, where one VP
is sched-out after hold a spinlock, then other VPs for same lock are sched-in
to waste the CPU time.

Our tests indicate that most spinlocks are held for less than 212 cycles.
Performance tests show that with 2X LP over-commitment we can get +2% perf
improvement for kernel build(Even more perf gain with more LPs).

Signed-off-by: Zhai Edwin <edwin.zhai@intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 50 insertions, 1 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 70020e505c2..a4580d65af5 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -61,6 +61,25 @@ module_param_named(unrestricted_guest,
 static int __read_mostly emulate_invalid_guest_state = 0;
 module_param(emulate_invalid_guest_state, bool, S_IRUGO);
 
+/*
+ * These 2 parameters are used to config the controls for Pause-Loop Exiting:
+ * ple_gap:    upper bound on the amount of time between two successive
+ *             executions of PAUSE in a loop. Also indicate if ple enabled.
+ *             According to test, this time is usually small than 41 cycles.
+ * ple_window: upper bound on the amount of time a guest is allowed to execute
+ *             in a PAUSE loop. Tests indicate that most spinlocks are held for
+ *             less than 2^12 cycles
+ * Time is measured based on a counter that runs at the same rate as the TSC,
+ * refer SDM volume 3b section 21.6.13 & 22.1.3.
+ */
+#define KVM_VMX_DEFAULT_PLE_GAP    41
+#define KVM_VMX_DEFAULT_PLE_WINDOW 4096
+static int ple_gap = KVM_VMX_DEFAULT_PLE_GAP;
+module_param(ple_gap, int, S_IRUGO);
+
+static int ple_window = KVM_VMX_DEFAULT_PLE_WINDOW;
+module_param(ple_window, int, S_IRUGO);
+
 struct vmcs {
         u32 revision_id;
         u32 abort;
@@ -319,6 +338,12 @@ static inline int cpu_has_vmx_unrestricted_guest(void)
                 SECONDARY_EXEC_UNRESTRICTED_GUEST;
 }
 
+static inline int cpu_has_vmx_ple(void)
+{
+        return vmcs_config.cpu_based_2nd_exec_ctrl &
+                SECONDARY_EXEC_PAUSE_LOOP_EXITING;
+}
+
 static inline int vm_need_virtualize_apic_accesses(struct kvm *kvm)
 {
         return flexpriority_enabled &&
@@ -1240,7 +1265,8 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
                         SECONDARY_EXEC_WBINVD_EXITING |
                         SECONDARY_EXEC_ENABLE_VPID |
                         SECONDARY_EXEC_ENABLE_EPT |
-                        SECONDARY_EXEC_UNRESTRICTED_GUEST;
+                        SECONDARY_EXEC_UNRESTRICTED_GUEST |
+                        SECONDARY_EXEC_PAUSE_LOOP_EXITING;
                 if (adjust_vmx_controls(min2, opt2,
                                         MSR_IA32_VMX_PROCBASED_CTLS2,
                                         &_cpu_based_2nd_exec_control) < 0)
@@ -1386,6 +1412,9 @@ static __init int hardware_setup(void)
         if (enable_ept && !cpu_has_vmx_ept_2m_page())
                 kvm_disable_largepages();
 
+        if (!cpu_has_vmx_ple())
+                ple_gap = 0;
+
         return alloc_kvm_area();
 }
 
@@ -2298,9 +2327,16 @@ static int vmx_vcpu_setup(struct vcpu_vmx *vmx)
                         exec_control &= ~SECONDARY_EXEC_ENABLE_EPT;
                 if (!enable_unrestricted_guest)
                         exec_control &= ~SECONDARY_EXEC_UNRESTRICTED_GUEST;
+                if (!ple_gap)
+                        exec_control &= ~SECONDARY_EXEC_PAUSE_LOOP_EXITING;
                 vmcs_write32(SECONDARY_VM_EXEC_CONTROL, exec_control);
         }
 
+        if (ple_gap) {
+                vmcs_write32(PLE_GAP, ple_gap);
+                vmcs_write32(PLE_WINDOW, ple_window);
+        }
+
         vmcs_write32(PAGE_FAULT_ERROR_CODE_MASK, !!bypass_guest_pf);
         vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, !!bypass_guest_pf);
         vmcs_write32(CR3_TARGET_COUNT, 0);           /* 22.2.1 */
@@ -3348,6 +3384,18 @@ out:
 }
 
 /*
+ * Indicate a busy-waiting vcpu in spinlock. We do not enable the PAUSE
+ * exiting, so only get here on cpu with PAUSE-Loop-Exiting.
+ */
+static int handle_pause(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
+{
+        skip_emulated_instruction(vcpu);
+        kvm_vcpu_on_spin(vcpu);
+
+        return 1;
+}
+
+/*
  * The exit handlers return 1 if the exit was handled fully and guest execution
  * may resume.  Otherwise they set the kvm_run parameter to indicate what needs
  * to be done to userspace and return 0.
@@ -3383,6 +3431,7 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
         [EXIT_REASON_MCE_DURING_VMENTRY]      = handle_machine_check,
         [EXIT_REASON_EPT_VIOLATION]           = handle_ept_violation,
         [EXIT_REASON_EPT_MISCONFIG]           = handle_ept_misconfig,
+        [EXIT_REASON_PAUSE_INSTRUCTION]       = handle_pause,
 };
 
 static const int kvm_vmx_max_exit_handlers =