[PATCH RESEND] KVM: x86 emulator: fix debug reg mov instructions

[Avi Kivity <avi@qumranet.com>]

More fallout from the writeback fixes: debug register transfer instructions
do their own writeback and thus need to disable the general writeback
mechanism.  This fixes oopses and some guest failures on AMD machines (the
Intel variant decodes the instruction in hardware and thus does not need
emulation).

Cc: Alistair John Strachan <alistair@devzero.co.uk>
Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
index 1f979cb..4b8a0cc 100644
--- a/drivers/kvm/x86_emulate.c
+++ b/drivers/kvm/x86_emulate.c
@@ -1217,11 +1217,13 @@ twobyte_insn:
 		}
 		break;
 	case 0x21: /* mov from dr to reg */
+		no_wb = 1;
 		if (modrm_mod != 3)
 			goto cannot_emulate;
 		rc = emulator_get_dr(ctxt, modrm_reg, &_regs[modrm_rm]);
 		break;
 	case 0x23: /* mov from reg to dr */
+		no_wb = 1;
 		if (modrm_mod != 3)
 			goto cannot_emulate;
 		rc = emulator_set_dr(ctxt, modrm_reg, _regs[modrm_rm]);