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>

1 files changed, 21 insertions, 0 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 5e8d411b0a81..47419d6031ea 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -71,6 +71,9 @@ module_param(vmm_exclusive, bool, S_IRUGO);
 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_xsetbv(struct kvm_vcpu *vcpu)
 
 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;
 }