[PATCH] KVM: x86: Add a fast path for sending virtual IPIs in x2APIC mode

[Sean Christopherson]

Add a fast path to handle writes to the ICR when the local APIC is
emulated in the kernel and x2APIC is enabled.  The fast path is invoked
at ->handle_exit_irqoff() to emulate only the effect of the ICR write
itself, i.e. the sending of IPIs.  Sending IPIs early in the VM-Exit
flow reduces the latency of virtual IPIs by avoiding the expensive bits
of transitioning from guest to host, e.g. reacquiring KVM's SRCU lock.

Suggested-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kvm/emulate.c |  1 -
 arch/x86/kvm/lapic.c   |  5 +++--
 arch/x86/kvm/lapic.h   | 25 +++++++++++++++++++++++++
 arch/x86/kvm/svm.c     |  3 +++
 arch/x86/kvm/vmx/vmx.c |  2 ++
 5 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 952d1a4f4d7e..8313234e7d64 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -19,7 +19,6 @@
  */
 
 #include <linux/kvm_host.h>
-#include "kvm_cache_regs.h"
 #include <asm/kvm_emulate.h>
 #include <linux/stringify.h>
 #include <asm/debugreg.h>
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 452cedd6382b..0f02820332d4 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2655,9 +2655,10 @@ int kvm_x2apic_msr_write(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 	if (reg == APIC_ICR2)
 		return 1;
 
-	/* if this is ICR write vector before command */
+	/* ICR writes are handled early by kvm_x2apic_fast_icr_write(). */
 	if (reg == APIC_ICR)
-		kvm_lapic_reg_write(apic, APIC_ICR2, (u32)(data >> 32));
+		return 0;
+
 	return kvm_lapic_reg_write(apic, reg, (u32)data);
 }
 
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index c1d77436126a..19fd2734d9e6 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -6,6 +6,8 @@
 
 #include <linux/kvm_host.h>
 
+#include "kvm_cache_regs.h"
+
 #define KVM_APIC_INIT		0
 #define KVM_APIC_SIPI		1
 #define KVM_APIC_LVT_NUM	6
@@ -245,4 +247,27 @@ static inline enum lapic_mode kvm_apic_mode(u64 apic_base)
 	return apic_base & (MSR_IA32_APICBASE_ENABLE | X2APIC_ENABLE);
 }
 
+/*
+ * Fast path for sending virtual IPIs immediately after VM-Exit.  Fault
+ * detection and injection, e.g. if x2apic is disabled, tracing and/or skipping
+ * of the emulated instruction are all handled in the standard WRMSR path,
+ * kvm_emulate_wrmsr().
+ */
+static inline void kvm_x2apic_fast_icr_write(struct kvm_vcpu *vcpu)
+{
+	struct kvm_lapic *apic = vcpu->arch.apic;
+	u64 data;
+
+	if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(apic))
+		return;
+
+	if (kvm_rcx_read(vcpu) != (APIC_BASE_MSR + (APIC_ICR >> 4)))
+		return;
+
+	data = kvm_read_edx_eax(vcpu);
+
+	kvm_lapic_reg_write(apic, APIC_ICR2, (u32)(data >> 32));
+	WARN_ON_ONCE(kvm_lapic_reg_write(apic, APIC_ICR, (u32)data));
+}
+
 #endif
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index d02a73a48461..713510210b29 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -6189,7 +6189,10 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
 
 static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
 {
+	struct vcpu_svm *svm = to_svm(vcpu);
 
+	if (svm->vmcb->control.exit_code && svm->vmcb->control.exit_info_1)
+		kvm_x2apic_fast_icr_write(vcpu);
 }
 
 static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 621142e55e28..82412c4085fc 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6231,6 +6231,8 @@ 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 (vmx->exit_reason == EXIT_REASON_MSR_WRITE)
+		kvm_x2apic_fast_icr_write(vcpu);
 }
 
 static bool vmx_has_emulated_msr(int index)
-- 
2.24.0