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KVM: APIC: avoid instruction emulation for EOI writes

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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>
This commit is contained in:
Kevin Tian
2011-08-30 13:56:17 +03:00
committed by Avi Kivity
parent 45133ecaae
commit 58fbbf26eb
4 changed files with 43 additions and 0 deletions
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21
arch/x86/kvm/vmx.c
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@@ -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;
}