KVM: X86: Provide a capability to disable MWAIT intercepts

Allowing a guest to execute MWAIT without interception enables a guest
to put a (physical) CPU into a power saving state, where it takes
longer to return from than what may be desired by the host.

Don't give a guest that power over a host by default. (Especially,
since nothing prevents a guest from using MWAIT even when it is not
advertised via CPUID.)

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Jan H. Schönherr <jschoenh@amazon.de>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/api.txt

@@ -4358,6 +4358,24 @@ enables QEMU to build error log and branch to guest kernel registered
 machine check handling routine. Without this capability KVM will
 branch to guests' 0x200 interrupt vector.
 
+7.13 KVM_CAP_X86_DISABLE_EXITS
+
+Architectures: x86
+Parameters: args[0] defines which exits are disabled
+Returns: 0 on success, -EINVAL when args[0] contains invalid exits
+
+Valid bits in args[0] are
+
+#define KVM_X86_DISABLE_EXITS_MWAIT            (1 << 0)
+
+Enabling this capability on a VM provides userspace with a way to no
+longer intercept some instructions for improved latency in some
+workloads, and is suggested when vCPUs are associated to dedicated
+physical CPUs.  More bits can be added in the future; userspace can
+just pass the KVM_CHECK_EXTENSION result to KVM_ENABLE_CAP to disable
+all such vmexits.
+
+
 8. Other capabilities.
 ----------------------
 
@@ -4470,15 +4488,6 @@ reserved.
     Both registers and addresses are 64-bits wide.
     It will be possible to run 64-bit or 32-bit guest code.
 
-8.8 KVM_CAP_X86_GUEST_MWAIT
-
-Architectures: x86
-
-This capability indicates that guest using memory monotoring instructions
-(MWAIT/MWAITX) to stop the virtual CPU will not cause a VM exit.  As such time
-spent while virtual CPU is halted in this way will then be accounted for as
-guest running time on the host (as opposed to e.g. HLT).
-
 8.9 KVM_CAP_ARM_USER_IRQ
 
 Architectures: arm, arm64

arch/x86/include/asm/kvm_host.h

@@ -811,6 +811,8 @@ struct kvm_arch {
 
 	gpa_t wall_clock;
 
+	bool mwait_in_guest;
+
 	bool ept_identity_pagetable_done;
 	gpa_t ept_identity_map_addr;
 

arch/x86/kvm/svm.c

@@ -1398,7 +1398,7 @@ static void init_vmcb(struct vcpu_svm *svm)
 	set_intercept(svm, INTERCEPT_XSETBV);
 	set_intercept(svm, INTERCEPT_RSM);
 
-	if (!kvm_mwait_in_guest()) {
+	if (!kvm_mwait_in_guest(svm->vcpu.kvm)) {
 		set_intercept(svm, INTERCEPT_MONITOR);
 		set_intercept(svm, INTERCEPT_MWAIT);
 	}

arch/x86/kvm/vmx.c

@@ -3746,13 +3746,11 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
 	      CPU_BASED_UNCOND_IO_EXITING |
 	      CPU_BASED_MOV_DR_EXITING |
 	      CPU_BASED_USE_TSC_OFFSETING |
+	      CPU_BASED_MWAIT_EXITING |
+	      CPU_BASED_MONITOR_EXITING |
 	      CPU_BASED_INVLPG_EXITING |
 	      CPU_BASED_RDPMC_EXITING;
 
-	if (!kvm_mwait_in_guest())
-		min |= CPU_BASED_MWAIT_EXITING |
-			CPU_BASED_MONITOR_EXITING;
-
 	opt = CPU_BASED_TPR_SHADOW |
 	      CPU_BASED_USE_MSR_BITMAPS |
 	      CPU_BASED_ACTIVATE_SECONDARY_CONTROLS;
@@ -5544,6 +5542,9 @@ static u32 vmx_exec_control(struct vcpu_vmx *vmx)
 		exec_control |= CPU_BASED_CR3_STORE_EXITING |
 				CPU_BASED_CR3_LOAD_EXITING  |
 				CPU_BASED_INVLPG_EXITING;
+	if (kvm_mwait_in_guest(vmx->vcpu.kvm))
+		exec_control &= ~(CPU_BASED_MWAIT_EXITING |
+				CPU_BASED_MONITOR_EXITING);
 	return exec_control;
 }
 

arch/x86/kvm/x86.c

@@ -2813,9 +2813,15 @@ static int msr_io(struct kvm_vcpu *vcpu, struct kvm_msrs __user *user_msrs,
 	return r;
 }
 
+static inline bool kvm_can_mwait_in_guest(void)
+{
+	return boot_cpu_has(X86_FEATURE_MWAIT) &&
+		!boot_cpu_has_bug(X86_BUG_MONITOR);
+}
+
 int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 {
-	int r;
+	int r = 0;
 
 	switch (ext) {
 	case KVM_CAP_IRQCHIP:
@@ -2871,8 +2877,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ADJUST_CLOCK:
 		r = KVM_CLOCK_TSC_STABLE;
 		break;
-	case KVM_CAP_X86_GUEST_MWAIT:
+	case KVM_CAP_X86_DISABLE_EXITS:
-		r = kvm_mwait_in_guest();
+		if(kvm_can_mwait_in_guest())
+			r |= KVM_X86_DISABLE_EXITS_MWAIT;
 		break;
 	case KVM_CAP_X86_SMM:
 		/* SMBASE is usually relocated above 1M on modern chipsets,
@@ -2913,7 +2920,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = KVM_X2APIC_API_VALID_FLAGS;
 		break;
 	default:
-		r = 0;
 		break;
 	}
 	return r;
@@ -4218,6 +4224,16 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 
 		r = 0;
 		break;
+	case KVM_CAP_X86_DISABLE_EXITS:
+		r = -EINVAL;
+		if (cap->args[0] & ~KVM_X86_DISABLE_VALID_EXITS)
+			break;
+
+		if ((cap->args[0] & KVM_X86_DISABLE_EXITS_MWAIT) &&
+			kvm_can_mwait_in_guest())
+			kvm->arch.mwait_in_guest = true;
+		r = 0;
+		break;
 	default:
 		r = -EINVAL;
 		break;

arch/x86/kvm/x86.h

@@ -2,8 +2,6 @@
 #ifndef ARCH_X86_KVM_X86_H
 #define ARCH_X86_KVM_X86_H
 
-#include <asm/processor.h>
-#include <asm/mwait.h>
 #include <linux/kvm_host.h>
 #include <asm/pvclock.h>
 #include "kvm_cache_regs.h"
@@ -266,10 +264,12 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec)
 	    __rem;						\
 	 })
 
-static inline bool kvm_mwait_in_guest(void)
+#define KVM_X86_DISABLE_EXITS_MWAIT          (1 << 0)
+#define KVM_X86_DISABLE_VALID_EXITS          (KVM_X86_DISABLE_EXITS_MWAIT)
+
+static inline bool kvm_mwait_in_guest(struct kvm *kvm)
 {
-	return boot_cpu_has(X86_FEATURE_MWAIT) &&
+	return kvm->arch.mwait_in_guest;
-		!boot_cpu_has_bug(X86_BUG_MONITOR);
 }
 
 #endif

include/uapi/linux/kvm.h

@@ -929,7 +929,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_S390_GS 140
 #define KVM_CAP_S390_AIS 141
 #define KVM_CAP_SPAPR_TCE_VFIO 142
-#define KVM_CAP_X86_GUEST_MWAIT 143
+#define KVM_CAP_X86_DISABLE_EXITS 143
 #define KVM_CAP_ARM_USER_IRQ 144
 #define KVM_CAP_S390_CMMA_MIGRATION 145
 #define KVM_CAP_PPC_FWNMI 146

tools/include/uapi/linux/kvm.h

@@ -924,7 +924,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_S390_GS 140
 #define KVM_CAP_S390_AIS 141
 #define KVM_CAP_SPAPR_TCE_VFIO 142
-#define KVM_CAP_X86_GUEST_MWAIT 143
+#define KVM_CAP_X86_DISABLE_EXITS 143
 #define KVM_CAP_ARM_USER_IRQ 144
 #define KVM_CAP_S390_CMMA_MIGRATION 145
 #define KVM_CAP_PPC_FWNMI 146