RE: [PATCH] KVM: APIC: avoid instruction emulation for EOI writes

[Tian, Kevin]

> From: Avi Kivity [mailto:a...@redhat.com]
> Sent: Monday, August 29, 2011 3:24 PM
> 
> On 08/29/2011 09:09 AM, Tian, Kevin wrote:
> >
> >   static int handle_apic_access(struct kvm_vcpu *vcpu)
> >   {
> > +   unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
> > +   int access_type, offset;
> > +
> > +   access_type = (exit_qualification>>  12)&  0xf;
> > +   offset = exit_qualification&  0xfff;
> > +   /*
> > +    * Sane guest uses MOV instead of string operations to
> > +    * write EOI, with written value not cared. So make a
> > +    * short-circuit here by avoiding heavy instruction
> > +    * emulation.
> > +    */
> > +   if ((access_type == 1)&&  (offset == APIC_EOI)) {
> 
> Please replace this 1 with a #define.
> 

updated.

--

    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.t...@intel.com>
    <Based on earlier work from>:
    Signed-off-by: Eddie Dong <eddie.d...@intel.com>
    Signed-off-by: Marcelo Tosatti <mtosa...@redhat.com>

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 57dcbd4..933187e 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -864,6 +864,15 @@ static int apic_mmio_write(struct kvm_io_device *this,
        return 0;
 }
 
+void kvm_lapic_set_eoi(struct kvm_vcpu *vcpu)
+{
+       struct kvm_lapic *apic = vcpu->arch.apic;
+
+       if (apic)
+               apic_set_eoi(vcpu->arch.apic);
+}
+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 @@ int kvm_get_apic_interrupt(struct kvm_vcpu *vcpu);
 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..34c094f 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -4538,8 +4538,25 @@ static int handle_xsetbv(struct kvm_vcpu *vcpu)
        return 1;
 }
 
+#define APIC_ACCESS_TYPE_W 1 /* Linear write access during inst execution */
 static int handle_apic_access(struct kvm_vcpu *vcpu)
 {
+       unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
+       int access_type, offset;
+
+       access_type = (exit_qualification >> 12) & 0xf;
+       offset = exit_qualification & 0xfff;
+       /*
+        * Sane guest uses MOV instead of string operations to
+        * write EOI, with written value not cared. So make a
+        * short-circuit here by avoiding heavy instruction 
+        * emulation.
+        */
+       if ((access_type == APIC_ACCESS_TYPE_W) && (offset == APIC_EOI)) {
+               kvm_lapic_set_eoi(vcpu);
+               skip_emulated_instruction(vcpu);
+               return 1;
+       }
        return emulate_instruction(vcpu, 0) == EMULATE_DONE;
 }

Thanks
Kevin

20110829_kvm_eoi_opt.patch
Description: 20110829_kvm_eoi_opt.patch