KVM: x86 emulator: Add sysenter emulation

Handle #UD intercept of the sysenter instruction in 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

@@ -1476,6 +1476,71 @@ emulate_syscall(struct x86_emulate_ctxt *ctxt)
 	return 0;
 }
 
+static int
+emulate_sysenter(struct x86_emulate_ctxt *ctxt)
+{
+	struct decode_cache *c = &ctxt->decode;
+	struct kvm_segment cs, ss;
+	u64 msr_data;
+
+	/* 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;
+	}
+
+	/* XXX sysenter/sysexit have not been tested in 64bit mode.
+	* Therefore, we inject an #UD.
+	*/
+	if (ctxt->mode == X86EMUL_MODE_PROT64)
+		return -1;
+
+	setup_syscalls_segments(ctxt, &cs, &ss);
+
+	kvm_x86_ops->get_msr(ctxt->vcpu, MSR_IA32_SYSENTER_CS, &msr_data);
+	switch (ctxt->mode) {
+	case X86EMUL_MODE_PROT32:
+		if ((msr_data & 0xfffc) == 0x0) {
+			kvm_inject_gp(ctxt->vcpu, 0);
+			return -1;
+		}
+		break;
+	case X86EMUL_MODE_PROT64:
+		if (msr_data == 0x0) {
+			kvm_inject_gp(ctxt->vcpu, 0);
+			return -1;
+		}
+		break;
+	}
+
+	ctxt->eflags &= ~(EFLG_VM | EFLG_IF | EFLG_RF);
+	cs.selector = (u16)msr_data;
+	cs.selector &= ~SELECTOR_RPL_MASK;
+	ss.selector = cs.selector + 8;
+	ss.selector &= ~SELECTOR_RPL_MASK;
+	if (ctxt->mode == X86EMUL_MODE_PROT64
+		|| is_long_mode(ctxt->vcpu)) {
+		cs.db = 0;
+		cs.l = 1;
+	}
+
+	kvm_x86_ops->set_segment(ctxt->vcpu, &cs, VCPU_SREG_CS);
+	kvm_x86_ops->set_segment(ctxt->vcpu, &ss, VCPU_SREG_SS);
+
+	kvm_x86_ops->get_msr(ctxt->vcpu, MSR_IA32_SYSENTER_EIP, &msr_data);
+	c->eip = msr_data;
+
+	kvm_x86_ops->get_msr(ctxt->vcpu, MSR_IA32_SYSENTER_ESP, &msr_data);
+	c->regs[VCPU_REGS_RSP] = msr_data;
+
+	return 0;
+}
+
 int
 x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 {
@@ -2144,7 +2209,10 @@ x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 		c->dst.type = OP_NONE;
 		break;
 	case 0x34:		/* sysenter */
+		if (emulate_sysenter(ctxt) == -1)
 			goto cannot_emulate;
+		else
+			goto writeback;
 		break;
 	case 0x35:		/* sysexit */
 		goto cannot_emulate;