KVM: x86 emulator: Add sysexit emulation

Handle #UD intercept of the sysexit instruction in 64bit mode returning to
32bit compat mode on an AMD host.
Setup the segment descriptors for CS and SS and the EIP/ESP registers
according to the manual.
Signed-off-by: Christoph Egger <christoph.egger@amd.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/x86_emulate.c

@@ -1541,6 +1541,73 @@ emulate_sysenter(struct x86_emulate_ctxt *ctxt)
 	return 0;
 }
 
+static int
+emulate_sysexit(struct x86_emulate_ctxt *ctxt)
+{
+	struct decode_cache *c = &ctxt->decode;
+	struct kvm_segment cs, ss;
+	u64 msr_data;
+	int usermode;
+
+	/* inject #UD if LOCK prefix is used */
+	if (c->lock_prefix)
+		return -1;
+
+	/* inject #GP if in real mode or paging is disabled */
+	if (ctxt->mode == X86EMUL_MODE_REAL
+		|| !(ctxt->vcpu->arch.cr0 & X86_CR0_PE)) {
+		kvm_inject_gp(ctxt->vcpu, 0);
+		return -1;
+	}
+
+	/* sysexit must be called from CPL 0 */
+	if (kvm_x86_ops->get_cpl(ctxt->vcpu) != 0) {
+		kvm_inject_gp(ctxt->vcpu, 0);
+		return -1;
+	}
+
+	setup_syscalls_segments(ctxt, &cs, &ss);
+
+	if ((c->rex_prefix & 0x8) != 0x0)
+		usermode = X86EMUL_MODE_PROT64;
+	else
+		usermode = X86EMUL_MODE_PROT32;
+
+	cs.dpl = 3;
+	ss.dpl = 3;
+	kvm_x86_ops->get_msr(ctxt->vcpu, MSR_IA32_SYSENTER_CS, &msr_data);
+	switch (usermode) {
+	case X86EMUL_MODE_PROT32:
+		cs.selector = (u16)(msr_data + 16);
+		if ((msr_data & 0xfffc) == 0x0) {
+			kvm_inject_gp(ctxt->vcpu, 0);
+			return -1;
+		}
+		ss.selector = (u16)(msr_data + 24);
+		break;
+	case X86EMUL_MODE_PROT64:
+		cs.selector = (u16)(msr_data + 32);
+		if (msr_data == 0x0) {
+			kvm_inject_gp(ctxt->vcpu, 0);
+			return -1;
+		}
+		ss.selector = cs.selector + 8;
+		cs.db = 0;
+		cs.l = 1;
+		break;
+	}
+	cs.selector |= SELECTOR_RPL_MASK;
+	ss.selector |= SELECTOR_RPL_MASK;
+
+	kvm_x86_ops->set_segment(ctxt->vcpu, &cs, VCPU_SREG_CS);
+	kvm_x86_ops->set_segment(ctxt->vcpu, &ss, VCPU_SREG_SS);
+
+	c->eip = ctxt->vcpu->arch.regs[VCPU_REGS_RDX];
+	c->regs[VCPU_REGS_RSP] = ctxt->vcpu->arch.regs[VCPU_REGS_RCX];
+
+	return 0;
+}
+
 int
 x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 {
@@ -2215,7 +2282,10 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 			goto writeback;
 		break;
 	case 0x35:		/* sysexit */
-		goto cannot_emulate;
+		if (emulate_sysexit(ctxt) == -1)
+			goto cannot_emulate;
+		else
+			goto writeback;
 		break;
 	case 0x40 ... 0x4f:	/* cmov */
 		c->dst.val = c->dst.orig_val = c->src.val;