KVM: x86 emulator: Fix segment loading in VM86

This fixes a regression introduced in commit 03ebebeb ("KVM: x86
emulator: Leave segment limit and attributs alone in real mode").

The mentioned commit changed the segment descriptors for both real mode
and VM86 to only update the segment base instead of creating a
completely new descriptor with limit 0xffff so that unreal mode keeps
working across a segment register reload.

This leads to an invalid segment descriptor in the eyes of VMX, which
seems to be okay for real mode because KVM will fix it up before the
next VM entry or emulate the state, but it doesn't do this if the guest
is in VM86, so we end up with:

  KVM: entry failed, hardware error 0x80000021

Fix this by effectively reverting commit 03ebebeb for VM86 and leaving
it only in place for real mode, which is where it's really needed.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Gleb Natapov <gleb@redhat.com>

arch/x86/kvm/emulate.c

@@ -1578,12 +1578,21 @@ static int load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
 
 	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 */
+	if (ctxt->mode == X86EMUL_MODE_REAL) {
+		/* set real mode segment descriptor (keep limit etc. for
+		 * unreal mode) */
 		ctxt->ops->get_segment(ctxt, &dummy, &seg_desc, NULL, seg);
 		set_desc_base(&seg_desc, selector << 4);
 		goto load;
+	} else if (seg <= VCPU_SREG_GS && ctxt->mode == X86EMUL_MODE_VM86) {
+		/* VM86 needs a clean new 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;
+		seg_desc.dpl = 3;
+		goto load;
 	}
 
 	rpl = selector & 3;