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Subject[PATCH 2/2] KVM: VMX: Skip GUEST_CR3 VMREAD+VMWRITE if the VMCS is up-to-date
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.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
arch/x86/kvm/vmx/vmx.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index b530950a9c2b..6de09f60edf3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3003,10 +3003,12 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)

if (is_guest_mode(vcpu))
skip_cr3 = true;
- 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
+ skip_cr3 = true; /* vmcs01.GUEST_CR3 is up-to-date. */
ept_load_pdptrs(vcpu);
}

--
2.22.0
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