x86/PV: split out dealing with DRn from privileged instruction handling

This is in preparation for using the generic emulator here.

Some care is needed temporarily to not unduly alter guest register
state: The local variable "res" can only go away once this code got
fully switched over to using x86_emulate().

Also switch to IS_ERR_VALUE() instead of (incorrectly) open coding it.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2343,6 +2343,26 @@ static int priv_op_write_cr(unsigned int
     return X86EMUL_UNHANDLEABLE;
 }
 
+static int priv_op_read_dr(unsigned int reg, unsigned long *val,
+                           struct x86_emulate_ctxt *ctxt)
+{
+    unsigned long res = do_get_debugreg(reg);
+
+    if ( IS_ERR_VALUE(res) )
+        return X86EMUL_UNHANDLEABLE;
+
+    *val = res;
+
+    return X86EMUL_OKAY;
+}
+
+static int priv_op_write_dr(unsigned int reg, unsigned long val,
+                            struct x86_emulate_ctxt *ctxt)
+{
+    return do_set_debugreg(reg, val) == 0
+           ? X86EMUL_OKAY : X86EMUL_UNHANDLEABLE;
+}
+
 static inline uint64_t guest_misc_enable(uint64_t val)
 {
     val &= ~(MSR_IA32_MISC_ENABLE_PERF_AVAIL |
@@ -2761,16 +2781,14 @@ static int emulate_privileged_op(struct
         break;
 
     case 0x21: /* MOV DR?,<reg> */ {
-        unsigned long res;
         opcode = insn_fetch(u8, code_base, eip, code_limit);
         if ( opcode < 0xc0 )
             goto fail;
         modrm_reg += ((opcode >> 3) & 7) + (lock << 3);
         modrm_rm  |= (opcode >> 0) & 7;
-        reg = decode_register(modrm_rm, regs, 0);
-        if ( (res = do_get_debugreg(modrm_reg)) > (unsigned long)-256 )
+        if ( priv_op_read_dr(modrm_reg, decode_register(modrm_rm, regs, 0),
+                             NULL) != X86EMUL_OKAY )
             goto fail;
-        *reg = res;
         break;
     }
 
@@ -2799,7 +2817,7 @@ static int emulate_privileged_op(struct
         modrm_reg += ((opcode >> 3) & 7) + (lock << 3);
         modrm_rm  |= (opcode >> 0) & 7;
         reg = decode_register(modrm_rm, regs, 0);
-        if ( do_set_debugreg(modrm_reg, *reg) != 0 )
+        if ( priv_op_write_dr(modrm_reg, *reg, NULL) != X86EMUL_OKAY )
             goto fail;
         break;