KVM: APIC: avoid instruction emulation for EOI writes
Instruction emulation for EOI writes can be skipped, since sane
guest simply uses MOV instead of string operations. This is a nice
improvement when guest doesn't support x2apic or hyper-V EOI
support.
a single VM bandwidth is observed with ~8% bandwidth improvement
(7.4Gbps->8Gbps), by saving ~5% cycles from EOI emulation.
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
<Based on earlier work from>:
Signed-off-by: Eddie Dong <eddie.dong@intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 57dcbd4..52645f2 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -864,6 +864,15 @@
return 0;
}
+void kvm_lapic_set_eoi(struct kvm_vcpu *vcpu)
+{
+ struct kvm_lapic *apic = vcpu->arch.apic;
+
+ if (apic)
+ apic_reg_write(vcpu->arch.apic, APIC_EOI, 0);
+}
+EXPORT_SYMBOL_GPL(kvm_lapic_set_eoi);
+
void kvm_free_lapic(struct kvm_vcpu *vcpu)
{
if (!vcpu->arch.apic)
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 52c9e6b..8287243 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -26,6 +26,7 @@
void kvm_lapic_reset(struct kvm_vcpu *vcpu);
u64 kvm_lapic_get_cr8(struct kvm_vcpu *vcpu);
void kvm_lapic_set_tpr(struct kvm_vcpu *vcpu, unsigned long cr8);
+void kvm_lapic_set_eoi(struct kvm_vcpu *vcpu);
void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value);
u64 kvm_lapic_get_base(struct kvm_vcpu *vcpu);
void kvm_apic_set_version(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 5e8d411..47419d6 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -71,6 +71,9 @@
static int __read_mostly yield_on_hlt = 1;
module_param(yield_on_hlt, bool, S_IRUGO);
+static int __read_mostly fasteoi = 1;
+module_param(fasteoi, bool, S_IRUGO);
+
/*
* If nested=1, nested virtualization is supported, i.e., guests may use
* VMX and be a hypervisor for its own guests. If nested=0, guests may not
@@ -4540,6 +4543,24 @@
static int handle_apic_access(struct kvm_vcpu *vcpu)
{
+ if (likely(fasteoi)) {
+ unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
+ int access_type, offset;
+
+ access_type = exit_qualification & APIC_ACCESS_TYPE;
+ offset = exit_qualification & APIC_ACCESS_OFFSET;
+ /*
+ * Sane guest uses MOV to write EOI, with written value
+ * not cared. So make a short-circuit here by avoiding
+ * heavy instruction emulation.
+ */
+ if ((access_type == TYPE_LINEAR_APIC_INST_WRITE) &&
+ (offset == APIC_EOI)) {
+ kvm_lapic_set_eoi(vcpu);
+ skip_emulated_instruction(vcpu);
+ return 1;
+ }
+ }
return emulate_instruction(vcpu, 0) == EMULATE_DONE;
}