Commit b17b7436 authored Sep 27, 2019 by Sean Christopherson Committed by Paolo Bonzini Oct 22, 2019

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



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>

parent 33af3a7e

arch/x86/kvm/vmx/vmx.c

+5 −3

Original line number	Diff line number	Diff line
		@@ -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);
		}

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