KVM: VMX: Treat UMIP as emulated if and only if the host doesn't have UMIP

Advertise UMIP as emulated if and only if the host doesn't natively
support UMIP, otherwise vmx_umip_emulated() is misleading when the host
_does_ support UMIP.  Of the four users of vmx_umip_emulated(), two
already check for native support, and the logic in vmx_set_cpu_caps() is
relevant if and only if UMIP isn't natively supported as UMIP is set in
KVM's caps by kvm_set_cpu_caps() when UMIP is present in hardware.

That leaves KVM's stuffing of X86_CR4_UMIP into the default cr4_fixed1
value enumerated for nested VMX.  In that case, checking for (lack of)
host support is actually a bug fix of sorts, as enumerating UMIP support
based solely on descriptor table exiting works only because KVM doesn't
sanity check MSR_IA32_VMX_CR4_FIXED1.  E.g. if a (very theoretical) host
supported UMIP in hardware but didn't allow UMIP+VMX, KVM would advertise
UMIP but not actually emulate UMIP.  Of course, KVM would explode long
before it could run a nested VM on said theoretical CPU, as KVM doesn't
modify host CR4 when enabling VMX, i.e. would load an "illegal" value into
vmcs.HOST_CR4.

Reported-by: Robert Hoo <robert.hu@intel.com>
Link: https://lore.kernel.org/all/20230310125718.1442088-2-robert.hu@intel.com
Link: https://lore.kernel.org/r/20230413231914.1482782-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/vmx/capabilities.h

@@ -152,8 +152,8 @@ static inline bool cpu_has_vmx_ept(void)
 
 static inline bool vmx_umip_emulated(void)
 {
-	return vmcs_config.cpu_based_2nd_exec_ctrl &
-		SECONDARY_EXEC_DESC;
+	return !boot_cpu_has(X86_FEATURE_UMIP) &&
+	       (vmcs_config.cpu_based_2nd_exec_ctrl & SECONDARY_EXEC_DESC);
 }
 
 static inline bool cpu_has_vmx_rdtscp(void)

arch/x86/kvm/vmx/nested.c

@@ -2328,8 +2328,7 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct loaded_vmcs *vmcs0
 		 * 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))
+		if (vmx_umip_emulated() && (vmcs12->guest_cr4 & X86_CR4_UMIP))
 			exec_control |= SECONDARY_EXEC_DESC;
 
 		if (exec_control & SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY)

arch/x86/kvm/vmx/vmx.c

@@ -3404,7 +3404,7 @@ void vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 	else
 		hw_cr4 |= KVM_PMODE_VM_CR4_ALWAYS_ON;
 
-	if (!boot_cpu_has(X86_FEATURE_UMIP) && vmx_umip_emulated()) {
+	if (vmx_umip_emulated()) {
 		if (cr4 & X86_CR4_UMIP) {
 			secondary_exec_controls_setbit(vmx, SECONDARY_EXEC_DESC);
 			hw_cr4 &= ~X86_CR4_UMIP;