[PATCH 40/43] KVM: nVMX: Preset *DT exiting in vmcs02 when emulating UMIP

[Paolo Bonzini]

From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

KVM dynamically toggles SECONDARY_EXEC_DESC to intercept (a subset of)
instructions that are subject to User-Mode Instruction Prevention, i.e.
VMCS.SECONDARY_EXEC_DESC == CR4.UMIP when emulating UMIP.  Preset the
VMCS control when preparing vmcs02 to avoid unnecessarily VMWRITEs,
e.g. KVM will clear VMCS.SECONDARY_EXEC_DESC in prepare_vmcs02_early()
and then set it in vmx_set_cr4().

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/vmx/nested.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 14a8cfade50f..598540ce0f52 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -2085,6 +2085,14 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
 		/* VMCS shadowing for L2 is emulated for now */
 		exec_control &= ~SECONDARY_EXEC_SHADOW_VMCS;
 
+		/*
+		 * Preset *DT exiting when emulating UMIP, so that vmx_set_cr4()
+		 * will not have to rewrite the controls just for this bit.
+		 */
+		if (!boot_cpu_has(X86_FEATURE_UMIP) && vmx_umip_emulated() &&
+		    (vmcs12->guest_cr4 & X86_CR4_UMIP))
+			exec_control |= SECONDARY_EXEC_DESC;
+
 		if (exec_control & SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY)
 			vmcs_write16(GUEST_INTR_STATUS,
 				vmcs12->guest_intr_status);
-- 
1.8.3.1