KVM: nVMX: Emulate HLT in L2 if it's not intercepted

JIRA: https://issues.redhat.com/browse/RHEL-95318

commit f43f7a2
Author: Sean Christopherson <seanjc@google.com>
Date:   Fri Jan 31 17:55:11 2025 -0800

    KVM: nVMX: Emulate HLT in L2 if it's not intercepted

    Extend VMX's nested intercept logic for emulated instructions to handle
    HLT interception, primarily for testing purposes.  Failure to allow
    emulation of HLT isn't all that interesting, as emulating HLT while L2 is
    active either requires forced emulation (and no #UD intercept in L1), TLB
    games in the guest to coerce KVM into emulating the wrong instruction, or
    a bug elsewhere in KVM.

    E.g. without commit 47ef3ef ("KVM: VMX: Handle event vectoring
    error in check_emulate_instruction()"), KVM can end up trying to emulate
    HLT if RIP happens to point at a HLT when a vectored event arrives with
    L2's IDT pointing at emulated MMIO.

    Note, vmx_check_intercept() is still broken when L1 wants to intercept an
    instruction, as KVM injects a #UD instead of synthesizing a nested VM-Exit.
    That issue extends far beyond HLT, punt on it for now.

    Link: https://lore.kernel.org/r/20250201015518.689704-5-seanjc@google.com
    Signed-off-by: Sean Christopherson <seanjc@google.com>

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: Maxim Levitsky

arch/x86/kvm/vmx/vmx.c

@@ -7995,6 +7995,11 @@ int vmx_check_intercept(struct kvm_vcpu *vcpu,
 		/* FIXME: produce nested vmexit and return X86EMUL_INTERCEPTED.  */
 		break;
 
+	case x86_intercept_hlt:
+		if (!nested_cpu_has(vmcs12, CPU_BASED_HLT_EXITING))
+			return X86EMUL_CONTINUE;
+		break;
+
 	case x86_intercept_pause:
 		/*
 		 * PAUSE is a single-byte NOP with a REPE prefix, i.e. collides