KVM: x86: Allow clearing RFLAGS.RF on forced emulation to test code #DBs

Extend force_emulation_prefix to an 'int' and use bit 1 as a flag to
indicate that KVM should clear RFLAGS.RF before emulating, e.g. to allow
tests to force emulation of code breakpoints in conjunction with MOV/POP
SS blocking, which is impossible without KVM intervention as VMX
unconditionally sets RFLAGS.RF on intercepted #UD.

Make the behavior controllable so that tests can also test RFLAGS.RF=1
(again in conjunction with code #DBs).

Note, clearing RFLAGS.RF won't create an infinite #DB loop as the guest's
IRET from the #DB handler will return to the instruction and not the
prefix, i.e. the restart won't force emulation.

Opportunistically convert the permissions to the preferred octal format.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220830231614.3580124-5-seanjc@google.comSigned-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -173,8 +173,13 @@ bool __read_mostly enable_vmware_backdoor = false;
 module_param(enable_vmware_backdoor, bool, S_IRUGO);
 EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
 
-static bool __read_mostly force_emulation_prefix = false;
-module_param(force_emulation_prefix, bool, S_IRUGO);
+/*
+ * Flags to manipulate forced emulation behavior (any non-zero value will
+ * enable forced emulation).
+ */
+#define KVM_FEP_CLEAR_RFLAGS_RF	BIT(1)
+static int __read_mostly force_emulation_prefix;
+module_param(force_emulation_prefix, int, 0444);
 
 int __read_mostly pi_inject_timer = -1;
 module_param(pi_inject_timer, bint, S_IRUGO | S_IWUSR);
@@ -7255,6 +7260,8 @@ int handle_ud(struct kvm_vcpu *vcpu)
 	    kvm_read_guest_virt(vcpu, kvm_get_linear_rip(vcpu),
 				sig, sizeof(sig), &e) == 0 &&
 	    memcmp(sig, kvm_emulate_prefix, sizeof(sig)) == 0) {
+		if (force_emulation_prefix & KVM_FEP_CLEAR_RFLAGS_RF)
+			kvm_set_rflags(vcpu, kvm_get_rflags(vcpu) & ~X86_EFLAGS_RF);
 		kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
 		emul_type = EMULTYPE_TRAP_UD_FORCED;
 	}