KVM: VMX: FIXED+PHYSICAL mode single target IPI fastpath

ICR and TSCDEADLINE MSRs write cause the main MSRs write vmexits in our
product observation, multicast IPIs are not as common as unicast IPI like
RESCHEDULE_VECTOR and CALL_FUNCTION_SINGLE_VECTOR etc.

This patch introduce a mechanism to handle certain performance-critical
WRMSRs in a very early stage of KVM VMExit handler.

This mechanism is specifically used for accelerating writes to x2APIC ICR
that attempt to send a virtual IPI with physical destination-mode, fixed
delivery-mode and single target. Which was found as one of the main causes
of VMExits for Linux workloads.

The reason this mechanism significantly reduce the latency of such virtual
IPIs is by sending the physical IPI to the target vCPU in a very early stage
of KVM VMExit handler, before host interrupts are enabled and before expensive
operations such as reacquiring KVM’s SRCU lock.
Latency is reduced even more when KVM is able to use APICv posted-interrupt
mechanism (which allows to deliver the virtual IPI directly to target vCPU
without the need to kick it to host).

Testing on Xeon Skylake server:

The virtual IPI latency from sender send to receiver receive reduces
more than 200+ cpu cycles.

Reviewed-by: Liran Alon <liran.alon@oracle.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -175,6 +175,11 @@ enum {
 	VCPU_SREG_LDTR,
 };
 
+enum exit_fastpath_completion {
+	EXIT_FASTPATH_NONE,
+	EXIT_FASTPATH_SKIP_EMUL_INS,
+};
+
 #include <asm/kvm_emulate.h>
 
 #define KVM_NR_MEM_OBJS 40
@@ -1095,7 +1100,8 @@ struct kvm_x86_ops {
 	void (*tlb_flush_gva)(struct kvm_vcpu *vcpu, gva_t addr);
 
 	void (*run)(struct kvm_vcpu *vcpu);
-	int (*handle_exit)(struct kvm_vcpu *vcpu);
+	int (*handle_exit)(struct kvm_vcpu *vcpu,
+		enum exit_fastpath_completion exit_fastpath);
 	int (*skip_emulated_instruction)(struct kvm_vcpu *vcpu);
 	void (*set_interrupt_shadow)(struct kvm_vcpu *vcpu, int mask);
 	u32 (*get_interrupt_shadow)(struct kvm_vcpu *vcpu);
@@ -1145,7 +1151,8 @@ struct kvm_x86_ops {
 	int (*check_intercept)(struct kvm_vcpu *vcpu,
 			       struct x86_instruction_info *info,
 			       enum x86_intercept_stage stage);
-	void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu);
+	void (*handle_exit_irqoff)(struct kvm_vcpu *vcpu,
+		enum exit_fastpath_completion *exit_fastpath);
 	bool (*mpx_supported)(void);
 	bool (*xsaves_supported)(void);
 	bool (*umip_emulated)(void);

arch/x86/kvm/svm.c

@@ -4935,7 +4935,8 @@ static void svm_get_exit_info(struct kvm_vcpu *vcpu, u64 *info1, u64 *info2)
 	*info2 = control->exit_info_2;
 }
 
-static int handle_exit(struct kvm_vcpu *vcpu)
+static int handle_exit(struct kvm_vcpu *vcpu,
+	enum exit_fastpath_completion exit_fastpath)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 	struct kvm_run *kvm_run = vcpu->run;
@@ -4993,7 +4994,10 @@ static int handle_exit(struct kvm_vcpu *vcpu)
 		       __func__, svm->vmcb->control.exit_int_info,
 		       exit_code);
 
-	if (exit_code >= ARRAY_SIZE(svm_exit_handlers)
+	if (exit_fastpath == EXIT_FASTPATH_SKIP_EMUL_INS) {
+		kvm_skip_emulated_instruction(vcpu);
+		return 1;
+	} else if (exit_code >= ARRAY_SIZE(svm_exit_handlers)
 	    || !svm_exit_handlers[exit_code]) {
 		vcpu_unimpl(vcpu, "svm: unexpected exit reason 0x%x\n", exit_code);
 		dump_vmcb(vcpu);
@@ -6186,9 +6190,12 @@ out:
 	return ret;
 }
 
-static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
+static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu,
+	enum exit_fastpath_completion *exit_fastpath)
 {
-
+	if (!is_guest_mode(vcpu) &&
+		to_svm(vcpu)->vmcb->control.exit_code == EXIT_REASON_MSR_WRITE)
+		*exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
 }
 
 static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu)

arch/x86/kvm/vmx/vmx.c

@@ -5814,7 +5814,8 @@ void dump_vmcs(void)
  * The guest has exited.  See if we can fix it or if we need userspace
  * assistance.
  */
-static int vmx_handle_exit(struct kvm_vcpu *vcpu)
+static int vmx_handle_exit(struct kvm_vcpu *vcpu,
+	enum exit_fastpath_completion exit_fastpath)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 	u32 exit_reason = vmx->exit_reason;
@@ -5900,7 +5901,10 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu)
 		}
 	}
 
-	if (exit_reason < kvm_vmx_max_exit_handlers
+	if (exit_fastpath == EXIT_FASTPATH_SKIP_EMUL_INS) {
+		kvm_skip_emulated_instruction(vcpu);
+		return 1;
+	} else if (exit_reason < kvm_vmx_max_exit_handlers
 	    && kvm_vmx_exit_handlers[exit_reason]) {
 #ifdef CONFIG_RETPOLINE
 		if (exit_reason == EXIT_REASON_MSR_WRITE)
@@ -6248,7 +6252,8 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu)
 }
 STACK_FRAME_NON_STANDARD(handle_external_interrupt_irqoff);
 
-static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
+static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu,
+	enum exit_fastpath_completion *exit_fastpath)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
@@ -6256,6 +6261,9 @@ static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
 		handle_external_interrupt_irqoff(vcpu);
 	else if (vmx->exit_reason == EXIT_REASON_EXCEPTION_NMI)
 		handle_exception_nmi_irqoff(vmx);
+	else if (!is_guest_mode(vcpu) &&
+		vmx->exit_reason == EXIT_REASON_MSR_WRITE)
+		*exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
 }
 
 static bool vmx_has_emulated_msr(int index)

arch/x86/kvm/x86.c

@@ -1525,6 +1525,49 @@ int kvm_emulate_wrmsr(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(kvm_emulate_wrmsr);
 
+/*
+ * The fast path for frequent and performance sensitive wrmsr emulation,
+ * i.e. the sending of IPI, sending IPI early in the VM-Exit flow reduces
+ * the latency of virtual IPI by avoiding the expensive bits of transitioning
+ * from guest to host, e.g. reacquiring KVM's SRCU lock. In contrast to the
+ * other cases which must be called after interrupts are enabled on the host.
+ */
+static int handle_fastpath_set_x2apic_icr_irqoff(struct kvm_vcpu *vcpu, u64 data)
+{
+	if (lapic_in_kernel(vcpu) && apic_x2apic_mode(vcpu->arch.apic) &&
+		((data & APIC_DEST_MASK) == APIC_DEST_PHYSICAL) &&
+		((data & APIC_MODE_MASK) == APIC_DM_FIXED)) {
+
+		kvm_lapic_set_reg(vcpu->arch.apic, APIC_ICR2, (u32)(data >> 32));
+		return kvm_lapic_reg_write(vcpu->arch.apic, APIC_ICR, (u32)data);
+	}
+
+	return 1;
+}
+
+enum exit_fastpath_completion handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
+{
+	u32 msr = kvm_rcx_read(vcpu);
+	u64 data = kvm_read_edx_eax(vcpu);
+	int ret = 0;
+
+	switch (msr) {
+	case APIC_BASE_MSR + (APIC_ICR >> 4):
+		ret = handle_fastpath_set_x2apic_icr_irqoff(vcpu, data);
+		break;
+	default:
+		return EXIT_FASTPATH_NONE;
+	}
+
+	if (!ret) {
+		trace_kvm_msr_write(msr, data);
+		return EXIT_FASTPATH_SKIP_EMUL_INS;
+	}
+
+	return EXIT_FASTPATH_NONE;
+}
+EXPORT_SYMBOL_GPL(handle_fastpath_set_msr_irqoff);
+
 /*
  * Adapt set_msr() to msr_io()'s calling convention
  */
@@ -7995,6 +8038,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	bool req_int_win =
 		dm_request_for_irq_injection(vcpu) &&
 		kvm_cpu_accept_dm_intr(vcpu);
+	enum exit_fastpath_completion exit_fastpath = EXIT_FASTPATH_NONE;
 
 	bool req_immediate_exit = false;
 
@@ -8241,7 +8285,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	vcpu->mode = OUTSIDE_GUEST_MODE;
 	smp_wmb();
 
-	kvm_x86_ops->handle_exit_irqoff(vcpu);
+	kvm_x86_ops->handle_exit_irqoff(vcpu, &exit_fastpath);
 
 	/*
 	 * Consume any pending interrupts, including the possible source of
@@ -8285,7 +8329,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		kvm_lapic_sync_from_vapic(vcpu);
 
 	vcpu->arch.gpa_available = false;
-	r = kvm_x86_ops->handle_exit(vcpu);
+	r = kvm_x86_ops->handle_exit(vcpu, exit_fastpath);
 	return r;
 
 cancel_injection:

arch/x86/kvm/x86.h

@@ -291,6 +291,7 @@ bool kvm_mtrr_check_gfn_range_consistency(struct kvm_vcpu *vcpu, gfn_t gfn,
 bool kvm_vector_hashing_enabled(void);
 int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 			    int emulation_type, void *insn, int insn_len);
+enum exit_fastpath_completion handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu);
 
 #define KVM_SUPPORTED_XCR0     (XFEATURE_MASK_FP | XFEATURE_MASK_SSE \
 				| XFEATURE_MASK_YMM | XFEATURE_MASK_BNDREGS \