KVM: x86 emulator: Emulate task switch in emulator.c
authorGleb Natapov <gleb@redhat.com>
Thu, 18 Mar 2010 13:20:17 +0000 (15:20 +0200)
committerAvi Kivity <avi@redhat.com>
Mon, 17 May 2010 09:16:15 +0000 (12:16 +0300)
Implement emulation of 16/32 bit task switch in emulator.c

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
arch/x86/include/asm/kvm_emulate.h
arch/x86/kvm/emulate.c

index f901467..bd46929 100644 (file)
@@ -11,6 +11,8 @@
 #ifndef _ASM_X86_KVM_X86_EMULATE_H
 #define _ASM_X86_KVM_X86_EMULATE_H
 
+#include <asm/desc_defs.h>
+
 struct x86_emulate_ctxt;
 
 /*
@@ -210,5 +212,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt,
                    struct x86_emulate_ops *ops);
 int x86_emulate_insn(struct x86_emulate_ctxt *ctxt,
                     struct x86_emulate_ops *ops);
+int emulator_task_switch(struct x86_emulate_ctxt *ctxt,
+                        struct x86_emulate_ops *ops,
+                        u16 tss_selector, int reason);
 
 #endif /* _ASM_X86_KVM_X86_EMULATE_H */
index d696cbd..db4776c 100644 (file)
@@ -33,6 +33,7 @@
 #include <asm/kvm_emulate.h>
 
 #include "x86.h"
+#include "tss.h"
 
 /*
  * Opcode effective-address decode tables.
@@ -1221,6 +1222,198 @@ done:
        return (rc == X86EMUL_UNHANDLEABLE) ? -1 : 0;
 }
 
+static u32 desc_limit_scaled(struct desc_struct *desc)
+{
+       u32 limit = get_desc_limit(desc);
+
+       return desc->g ? (limit << 12) | 0xfff : limit;
+}
+
+static void get_descriptor_table_ptr(struct x86_emulate_ctxt *ctxt,
+                                    struct x86_emulate_ops *ops,
+                                    u16 selector, struct desc_ptr *dt)
+{
+       if (selector & 1 << 2) {
+               struct desc_struct desc;
+               memset (dt, 0, sizeof *dt);
+               if (!ops->get_cached_descriptor(&desc, VCPU_SREG_LDTR, ctxt->vcpu))
+                       return;
+
+               dt->size = desc_limit_scaled(&desc); /* what if limit > 65535? */
+               dt->address = get_desc_base(&desc);
+       } else
+               ops->get_gdt(dt, ctxt->vcpu);
+}
+
+/* allowed just for 8 bytes segments */
+static int read_segment_descriptor(struct x86_emulate_ctxt *ctxt,
+                                  struct x86_emulate_ops *ops,
+                                  u16 selector, struct desc_struct *desc)
+{
+       struct desc_ptr dt;
+       u16 index = selector >> 3;
+       int ret;
+       u32 err;
+       ulong addr;
+
+       get_descriptor_table_ptr(ctxt, ops, selector, &dt);
+
+       if (dt.size < index * 8 + 7) {
+               kvm_inject_gp(ctxt->vcpu, selector & 0xfffc);
+               return X86EMUL_PROPAGATE_FAULT;
+       }
+       addr = dt.address + index * 8;
+       ret = ops->read_std(addr, desc, sizeof *desc, ctxt->vcpu,  &err);
+       if (ret == X86EMUL_PROPAGATE_FAULT)
+               kvm_inject_page_fault(ctxt->vcpu, addr, err);
+
+       return ret;
+}
+
+/* allowed just for 8 bytes segments */
+static int write_segment_descriptor(struct x86_emulate_ctxt *ctxt,
+                                   struct x86_emulate_ops *ops,
+                                   u16 selector, struct desc_struct *desc)
+{
+       struct desc_ptr dt;
+       u16 index = selector >> 3;
+       u32 err;
+       ulong addr;
+       int ret;
+
+       get_descriptor_table_ptr(ctxt, ops, selector, &dt);
+
+       if (dt.size < index * 8 + 7) {
+               kvm_inject_gp(ctxt->vcpu, selector & 0xfffc);
+               return X86EMUL_PROPAGATE_FAULT;
+       }
+
+       addr = dt.address + index * 8;
+       ret = ops->write_std(addr, desc, sizeof *desc, ctxt->vcpu, &err);
+       if (ret == X86EMUL_PROPAGATE_FAULT)
+               kvm_inject_page_fault(ctxt->vcpu, addr, err);
+
+       return ret;
+}
+
+static int load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
+                                  struct x86_emulate_ops *ops,
+                                  u16 selector, int seg)
+{
+       struct desc_struct seg_desc;
+       u8 dpl, rpl, cpl;
+       unsigned err_vec = GP_VECTOR;
+       u32 err_code = 0;
+       bool null_selector = !(selector & ~0x3); /* 0000-0003 are null */
+       int ret;
+
+       memset(&seg_desc, 0, sizeof seg_desc);
+
+       if ((seg <= VCPU_SREG_GS && ctxt->mode == X86EMUL_MODE_VM86)
+           || ctxt->mode == X86EMUL_MODE_REAL) {
+               /* set real mode segment descriptor */
+               set_desc_base(&seg_desc, selector << 4);
+               set_desc_limit(&seg_desc, 0xffff);
+               seg_desc.type = 3;
+               seg_desc.p = 1;
+               seg_desc.s = 1;
+               goto load;
+       }
+
+       /* NULL selector is not valid for TR, CS and SS */
+       if ((seg == VCPU_SREG_CS || seg == VCPU_SREG_SS || seg == VCPU_SREG_TR)
+           && null_selector)
+               goto exception;
+
+       /* TR should be in GDT only */
+       if (seg == VCPU_SREG_TR && (selector & (1 << 2)))
+               goto exception;
+
+       if (null_selector) /* for NULL selector skip all following checks */
+               goto load;
+
+       ret = read_segment_descriptor(ctxt, ops, selector, &seg_desc);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+
+       err_code = selector & 0xfffc;
+       err_vec = GP_VECTOR;
+
+       /* can't load system descriptor into segment selecor */
+       if (seg <= VCPU_SREG_GS && !seg_desc.s)
+               goto exception;
+
+       if (!seg_desc.p) {
+               err_vec = (seg == VCPU_SREG_SS) ? SS_VECTOR : NP_VECTOR;
+               goto exception;
+       }
+
+       rpl = selector & 3;
+       dpl = seg_desc.dpl;
+       cpl = ops->cpl(ctxt->vcpu);
+
+       switch (seg) {
+       case VCPU_SREG_SS:
+               /*
+                * segment is not a writable data segment or segment
+                * selector's RPL != CPL or segment selector's RPL != CPL
+                */
+               if (rpl != cpl || (seg_desc.type & 0xa) != 0x2 || dpl != cpl)
+                       goto exception;
+               break;
+       case VCPU_SREG_CS:
+               if (!(seg_desc.type & 8))
+                       goto exception;
+
+               if (seg_desc.type & 4) {
+                       /* conforming */
+                       if (dpl > cpl)
+                               goto exception;
+               } else {
+                       /* nonconforming */
+                       if (rpl > cpl || dpl != cpl)
+                               goto exception;
+               }
+               /* CS(RPL) <- CPL */
+               selector = (selector & 0xfffc) | cpl;
+               break;
+       case VCPU_SREG_TR:
+               if (seg_desc.s || (seg_desc.type != 1 && seg_desc.type != 9))
+                       goto exception;
+               break;
+       case VCPU_SREG_LDTR:
+               if (seg_desc.s || seg_desc.type != 2)
+                       goto exception;
+               break;
+       default: /*  DS, ES, FS, or GS */
+               /*
+                * segment is not a data or readable code segment or
+                * ((segment is a data or nonconforming code segment)
+                * and (both RPL and CPL > DPL))
+                */
+               if ((seg_desc.type & 0xa) == 0x8 ||
+                   (((seg_desc.type & 0xc) != 0xc) &&
+                    (rpl > dpl && cpl > dpl)))
+                       goto exception;
+               break;
+       }
+
+       if (seg_desc.s) {
+               /* mark segment as accessed */
+               seg_desc.type |= 1;
+               ret = write_segment_descriptor(ctxt, ops, selector, &seg_desc);
+               if (ret != X86EMUL_CONTINUE)
+                       return ret;
+       }
+load:
+       ops->set_segment_selector(selector, seg, ctxt->vcpu);
+       ops->set_cached_descriptor(&seg_desc, seg, ctxt->vcpu);
+       return X86EMUL_CONTINUE;
+exception:
+       kvm_queue_exception_e(ctxt->vcpu, err_vec, err_code);
+       return X86EMUL_PROPAGATE_FAULT;
+}
+
 static inline void emulate_push(struct x86_emulate_ctxt *ctxt)
 {
        struct decode_cache *c = &ctxt->decode;
@@ -1812,6 +2005,376 @@ static bool emulator_io_permited(struct x86_emulate_ctxt *ctxt,
        return true;
 }
 
+static u32 get_cached_descriptor_base(struct x86_emulate_ctxt *ctxt,
+                                     struct x86_emulate_ops *ops,
+                                     int seg)
+{
+       struct desc_struct desc;
+       if (ops->get_cached_descriptor(&desc, seg, ctxt->vcpu))
+               return get_desc_base(&desc);
+       else
+               return ~0;
+}
+
+static void save_state_to_tss16(struct x86_emulate_ctxt *ctxt,
+                               struct x86_emulate_ops *ops,
+                               struct tss_segment_16 *tss)
+{
+       struct decode_cache *c = &ctxt->decode;
+
+       tss->ip = c->eip;
+       tss->flag = ctxt->eflags;
+       tss->ax = c->regs[VCPU_REGS_RAX];
+       tss->cx = c->regs[VCPU_REGS_RCX];
+       tss->dx = c->regs[VCPU_REGS_RDX];
+       tss->bx = c->regs[VCPU_REGS_RBX];
+       tss->sp = c->regs[VCPU_REGS_RSP];
+       tss->bp = c->regs[VCPU_REGS_RBP];
+       tss->si = c->regs[VCPU_REGS_RSI];
+       tss->di = c->regs[VCPU_REGS_RDI];
+
+       tss->es = ops->get_segment_selector(VCPU_SREG_ES, ctxt->vcpu);
+       tss->cs = ops->get_segment_selector(VCPU_SREG_CS, ctxt->vcpu);
+       tss->ss = ops->get_segment_selector(VCPU_SREG_SS, ctxt->vcpu);
+       tss->ds = ops->get_segment_selector(VCPU_SREG_DS, ctxt->vcpu);
+       tss->ldt = ops->get_segment_selector(VCPU_SREG_LDTR, ctxt->vcpu);
+}
+
+static int load_state_from_tss16(struct x86_emulate_ctxt *ctxt,
+                                struct x86_emulate_ops *ops,
+                                struct tss_segment_16 *tss)
+{
+       struct decode_cache *c = &ctxt->decode;
+       int ret;
+
+       c->eip = tss->ip;
+       ctxt->eflags = tss->flag | 2;
+       c->regs[VCPU_REGS_RAX] = tss->ax;
+       c->regs[VCPU_REGS_RCX] = tss->cx;
+       c->regs[VCPU_REGS_RDX] = tss->dx;
+       c->regs[VCPU_REGS_RBX] = tss->bx;
+       c->regs[VCPU_REGS_RSP] = tss->sp;
+       c->regs[VCPU_REGS_RBP] = tss->bp;
+       c->regs[VCPU_REGS_RSI] = tss->si;
+       c->regs[VCPU_REGS_RDI] = tss->di;
+
+       /*
+        * SDM says that segment selectors are loaded before segment
+        * descriptors
+        */
+       ops->set_segment_selector(tss->ldt, VCPU_SREG_LDTR, ctxt->vcpu);
+       ops->set_segment_selector(tss->es, VCPU_SREG_ES, ctxt->vcpu);
+       ops->set_segment_selector(tss->cs, VCPU_SREG_CS, ctxt->vcpu);
+       ops->set_segment_selector(tss->ss, VCPU_SREG_SS, ctxt->vcpu);
+       ops->set_segment_selector(tss->ds, VCPU_SREG_DS, ctxt->vcpu);
+
+       /*
+        * Now load segment descriptors. If fault happenes at this stage
+        * it is handled in a context of new task
+        */
+       ret = load_segment_descriptor(ctxt, ops, tss->ldt, VCPU_SREG_LDTR);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->es, VCPU_SREG_ES);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->cs, VCPU_SREG_CS);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->ss, VCPU_SREG_SS);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->ds, VCPU_SREG_DS);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+
+       return X86EMUL_CONTINUE;
+}
+
+static int task_switch_16(struct x86_emulate_ctxt *ctxt,
+                         struct x86_emulate_ops *ops,
+                         u16 tss_selector, u16 old_tss_sel,
+                         ulong old_tss_base, struct desc_struct *new_desc)
+{
+       struct tss_segment_16 tss_seg;
+       int ret;
+       u32 err, new_tss_base = get_desc_base(new_desc);
+
+       ret = ops->read_std(old_tss_base, &tss_seg, sizeof tss_seg, ctxt->vcpu,
+                           &err);
+       if (ret == X86EMUL_PROPAGATE_FAULT) {
+               /* FIXME: need to provide precise fault address */
+               kvm_inject_page_fault(ctxt->vcpu, old_tss_base, err);
+               return ret;
+       }
+
+       save_state_to_tss16(ctxt, ops, &tss_seg);
+
+       ret = ops->write_std(old_tss_base, &tss_seg, sizeof tss_seg, ctxt->vcpu,
+                            &err);
+       if (ret == X86EMUL_PROPAGATE_FAULT) {
+               /* FIXME: need to provide precise fault address */
+               kvm_inject_page_fault(ctxt->vcpu, old_tss_base, err);
+               return ret;
+       }
+
+       ret = ops->read_std(new_tss_base, &tss_seg, sizeof tss_seg, ctxt->vcpu,
+                           &err);
+       if (ret == X86EMUL_PROPAGATE_FAULT) {
+               /* FIXME: need to provide precise fault address */
+               kvm_inject_page_fault(ctxt->vcpu, new_tss_base, err);
+               return ret;
+       }
+
+       if (old_tss_sel != 0xffff) {
+               tss_seg.prev_task_link = old_tss_sel;
+
+               ret = ops->write_std(new_tss_base,
+                                    &tss_seg.prev_task_link,
+                                    sizeof tss_seg.prev_task_link,
+                                    ctxt->vcpu, &err);
+               if (ret == X86EMUL_PROPAGATE_FAULT) {
+                       /* FIXME: need to provide precise fault address */
+                       kvm_inject_page_fault(ctxt->vcpu, new_tss_base, err);
+                       return ret;
+               }
+       }
+
+       return load_state_from_tss16(ctxt, ops, &tss_seg);
+}
+
+static void save_state_to_tss32(struct x86_emulate_ctxt *ctxt,
+                               struct x86_emulate_ops *ops,
+                               struct tss_segment_32 *tss)
+{
+       struct decode_cache *c = &ctxt->decode;
+
+       tss->cr3 = ops->get_cr(3, ctxt->vcpu);
+       tss->eip = c->eip;
+       tss->eflags = ctxt->eflags;
+       tss->eax = c->regs[VCPU_REGS_RAX];
+       tss->ecx = c->regs[VCPU_REGS_RCX];
+       tss->edx = c->regs[VCPU_REGS_RDX];
+       tss->ebx = c->regs[VCPU_REGS_RBX];
+       tss->esp = c->regs[VCPU_REGS_RSP];
+       tss->ebp = c->regs[VCPU_REGS_RBP];
+       tss->esi = c->regs[VCPU_REGS_RSI];
+       tss->edi = c->regs[VCPU_REGS_RDI];
+
+       tss->es = ops->get_segment_selector(VCPU_SREG_ES, ctxt->vcpu);
+       tss->cs = ops->get_segment_selector(VCPU_SREG_CS, ctxt->vcpu);
+       tss->ss = ops->get_segment_selector(VCPU_SREG_SS, ctxt->vcpu);
+       tss->ds = ops->get_segment_selector(VCPU_SREG_DS, ctxt->vcpu);
+       tss->fs = ops->get_segment_selector(VCPU_SREG_FS, ctxt->vcpu);
+       tss->gs = ops->get_segment_selector(VCPU_SREG_GS, ctxt->vcpu);
+       tss->ldt_selector = ops->get_segment_selector(VCPU_SREG_LDTR, ctxt->vcpu);
+}
+
+static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt,
+                                struct x86_emulate_ops *ops,
+                                struct tss_segment_32 *tss)
+{
+       struct decode_cache *c = &ctxt->decode;
+       int ret;
+
+       ops->set_cr(3, tss->cr3, ctxt->vcpu);
+       c->eip = tss->eip;
+       ctxt->eflags = tss->eflags | 2;
+       c->regs[VCPU_REGS_RAX] = tss->eax;
+       c->regs[VCPU_REGS_RCX] = tss->ecx;
+       c->regs[VCPU_REGS_RDX] = tss->edx;
+       c->regs[VCPU_REGS_RBX] = tss->ebx;
+       c->regs[VCPU_REGS_RSP] = tss->esp;
+       c->regs[VCPU_REGS_RBP] = tss->ebp;
+       c->regs[VCPU_REGS_RSI] = tss->esi;
+       c->regs[VCPU_REGS_RDI] = tss->edi;
+
+       /*
+        * SDM says that segment selectors are loaded before segment
+        * descriptors
+        */
+       ops->set_segment_selector(tss->ldt_selector, VCPU_SREG_LDTR, ctxt->vcpu);
+       ops->set_segment_selector(tss->es, VCPU_SREG_ES, ctxt->vcpu);
+       ops->set_segment_selector(tss->cs, VCPU_SREG_CS, ctxt->vcpu);
+       ops->set_segment_selector(tss->ss, VCPU_SREG_SS, ctxt->vcpu);
+       ops->set_segment_selector(tss->ds, VCPU_SREG_DS, ctxt->vcpu);
+       ops->set_segment_selector(tss->fs, VCPU_SREG_FS, ctxt->vcpu);
+       ops->set_segment_selector(tss->gs, VCPU_SREG_GS, ctxt->vcpu);
+
+       /*
+        * Now load segment descriptors. If fault happenes at this stage
+        * it is handled in a context of new task
+        */
+       ret = load_segment_descriptor(ctxt, ops, tss->ldt_selector, VCPU_SREG_LDTR);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->es, VCPU_SREG_ES);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->cs, VCPU_SREG_CS);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->ss, VCPU_SREG_SS);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->ds, VCPU_SREG_DS);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->fs, VCPU_SREG_FS);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = load_segment_descriptor(ctxt, ops, tss->gs, VCPU_SREG_GS);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+
+       return X86EMUL_CONTINUE;
+}
+
+static int task_switch_32(struct x86_emulate_ctxt *ctxt,
+                         struct x86_emulate_ops *ops,
+                         u16 tss_selector, u16 old_tss_sel,
+                         ulong old_tss_base, struct desc_struct *new_desc)
+{
+       struct tss_segment_32 tss_seg;
+       int ret;
+       u32 err, new_tss_base = get_desc_base(new_desc);
+
+       ret = ops->read_std(old_tss_base, &tss_seg, sizeof tss_seg, ctxt->vcpu,
+                           &err);
+       if (ret == X86EMUL_PROPAGATE_FAULT) {
+               /* FIXME: need to provide precise fault address */
+               kvm_inject_page_fault(ctxt->vcpu, old_tss_base, err);
+               return ret;
+       }
+
+       save_state_to_tss32(ctxt, ops, &tss_seg);
+
+       ret = ops->write_std(old_tss_base, &tss_seg, sizeof tss_seg, ctxt->vcpu,
+                            &err);
+       if (ret == X86EMUL_PROPAGATE_FAULT) {
+               /* FIXME: need to provide precise fault address */
+               kvm_inject_page_fault(ctxt->vcpu, old_tss_base, err);
+               return ret;
+       }
+
+       ret = ops->read_std(new_tss_base, &tss_seg, sizeof tss_seg, ctxt->vcpu,
+                           &err);
+       if (ret == X86EMUL_PROPAGATE_FAULT) {
+               /* FIXME: need to provide precise fault address */
+               kvm_inject_page_fault(ctxt->vcpu, new_tss_base, err);
+               return ret;
+       }
+
+       if (old_tss_sel != 0xffff) {
+               tss_seg.prev_task_link = old_tss_sel;
+
+               ret = ops->write_std(new_tss_base,
+                                    &tss_seg.prev_task_link,
+                                    sizeof tss_seg.prev_task_link,
+                                    ctxt->vcpu, &err);
+               if (ret == X86EMUL_PROPAGATE_FAULT) {
+                       /* FIXME: need to provide precise fault address */
+                       kvm_inject_page_fault(ctxt->vcpu, new_tss_base, err);
+                       return ret;
+               }
+       }
+
+       return load_state_from_tss32(ctxt, ops, &tss_seg);
+}
+
+static int emulator_do_task_switch(struct x86_emulate_ctxt *ctxt,
+                                   struct x86_emulate_ops *ops,
+                                   u16 tss_selector, int reason)
+{
+       struct desc_struct curr_tss_desc, next_tss_desc;
+       int ret;
+       u16 old_tss_sel = ops->get_segment_selector(VCPU_SREG_TR, ctxt->vcpu);
+       ulong old_tss_base =
+               get_cached_descriptor_base(ctxt, ops, VCPU_SREG_TR);
+
+       /* FIXME: old_tss_base == ~0 ? */
+
+       ret = read_segment_descriptor(ctxt, ops, tss_selector, &next_tss_desc);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+       ret = read_segment_descriptor(ctxt, ops, old_tss_sel, &curr_tss_desc);
+       if (ret != X86EMUL_CONTINUE)
+               return ret;
+
+       /* FIXME: check that next_tss_desc is tss */
+
+       if (reason != TASK_SWITCH_IRET) {
+               if ((tss_selector & 3) > next_tss_desc.dpl ||
+                   ops->cpl(ctxt->vcpu) > next_tss_desc.dpl) {
+                       kvm_inject_gp(ctxt->vcpu, 0);
+                       return X86EMUL_PROPAGATE_FAULT;
+               }
+       }
+
+       if (!next_tss_desc.p || desc_limit_scaled(&next_tss_desc) < 0x67) {
+               kvm_queue_exception_e(ctxt->vcpu, TS_VECTOR,
+                                     tss_selector & 0xfffc);
+               return X86EMUL_PROPAGATE_FAULT;
+       }
+
+       if (reason == TASK_SWITCH_IRET || reason == TASK_SWITCH_JMP) {
+               curr_tss_desc.type &= ~(1 << 1); /* clear busy flag */
+               write_segment_descriptor(ctxt, ops, old_tss_sel,
+                                        &curr_tss_desc);
+       }
+
+       if (reason == TASK_SWITCH_IRET)
+               ctxt->eflags = ctxt->eflags & ~X86_EFLAGS_NT;
+
+       /* set back link to prev task only if NT bit is set in eflags
+          note that old_tss_sel is not used afetr this point */
+       if (reason != TASK_SWITCH_CALL && reason != TASK_SWITCH_GATE)
+               old_tss_sel = 0xffff;
+
+       if (next_tss_desc.type & 8)
+               ret = task_switch_32(ctxt, ops, tss_selector, old_tss_sel,
+                                    old_tss_base, &next_tss_desc);
+       else
+               ret = task_switch_16(ctxt, ops, tss_selector, old_tss_sel,
+                                    old_tss_base, &next_tss_desc);
+
+       if (reason == TASK_SWITCH_CALL || reason == TASK_SWITCH_GATE)
+               ctxt->eflags = ctxt->eflags | X86_EFLAGS_NT;
+
+       if (reason != TASK_SWITCH_IRET) {
+               next_tss_desc.type |= (1 << 1); /* set busy flag */
+               write_segment_descriptor(ctxt, ops, tss_selector,
+                                        &next_tss_desc);
+       }
+
+       ops->set_cr(0,  ops->get_cr(0, ctxt->vcpu) | X86_CR0_TS, ctxt->vcpu);
+       ops->set_cached_descriptor(&next_tss_desc, VCPU_SREG_TR, ctxt->vcpu);
+       ops->set_segment_selector(tss_selector, VCPU_SREG_TR, ctxt->vcpu);
+
+       return ret;
+}
+
+int emulator_task_switch(struct x86_emulate_ctxt *ctxt,
+                        struct x86_emulate_ops *ops,
+                        u16 tss_selector, int reason)
+{
+       struct decode_cache *c = &ctxt->decode;
+       int rc;
+
+       memset(c, 0, sizeof(struct decode_cache));
+       c->eip = ctxt->eip;
+       memcpy(c->regs, ctxt->vcpu->arch.regs, sizeof c->regs);
+
+       rc = emulator_do_task_switch(ctxt, ops, tss_selector, reason);
+
+       if (rc == X86EMUL_CONTINUE) {
+               memcpy(ctxt->vcpu->arch.regs, c->regs, sizeof c->regs);
+               kvm_rip_write(ctxt->vcpu, c->eip);
+       }
+
+       return rc;
+}
+
 int
 x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 {