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KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date

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Skip the VMWRITE to update GUEST_CR3 if CR3 is not available, i.e. has
not been read from the VMCS since the last VM-Enter.  If vcpu->arch.cr3
is stale, kvm_read_cr3(vcpu) will refresh vcpu->arch.cr3 from the VMCS,
meaning KVM will do a VMREAD and then VMWRITE the value it just pulled
from the VMCS.

Note, this is a purely theoretical change, no instances of skipping
the VMREAD+VMWRITE have been observed with this change.

Tested-by: Reto Buerki <reet@codelabs.ch>
Tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2019-09-27 14:45:17 -07:00 committed by Paolo Bonzini
parent 33af3a7ef9
commit b17b7436f2
1 changed files with 5 additions and 3 deletions
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8
arch/x86/kvm/vmx/vmx.c
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@ -3009,10 +3009,12 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
/* Loading vmcs02.GUEST_CR3 is handled by nested VM-Enter. */
if (is_guest_mode(vcpu))
update_guest_cr3 = false;
else if (enable_unrestricted_guest || is_paging(vcpu))
guest_cr3 = kvm_read_cr3(vcpu);
else
else if (!enable_unrestricted_guest && !is_paging(vcpu))
guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
else if (test_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_avail))
guest_cr3 = vcpu->arch.cr3;
else /* vmcs01.GUEST_CR3 is already up-to-date. */
update_guest_cr3 = false;
ept_load_pdptrs(vcpu);
}