gdbstub: x86: Support for setting segment registers

This allows to set segment registers via gdb also in system emulation
mode. Basic sanity checks are applied and nothing is changed if they
fail. But screwing up the target via this interface will never be
complicated, so I avoided being too paranoid here.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

gdbstub.c

@@ -563,6 +563,31 @@ static int cpu_gdb_read_register(CPUState *env, uint8_t *mem_buf, int n)
     return 0;
 }
 
+static int cpu_x86_gdb_load_seg(CPUState *env, int sreg, uint8_t *mem_buf)
+{
+    uint16_t selector = ldl_p(mem_buf);
+
+    if (selector != env->segs[sreg].selector) {
+#if defined(CONFIG_USER_ONLY)
+        cpu_x86_load_seg(env, sreg, selector);
+#else
+        unsigned int limit, flags;
+        target_ulong base;
+
+        if (!(env->cr[0] & CR0_PE_MASK) || (env->eflags & VM_MASK)) {
+            base = selector << 4;
+            limit = 0xffff;
+            flags = 0;
+        } else {
+            if (!cpu_x86_get_descr_debug(env, selector, &base, &limit, &flags))
+                return 4;
+        }
+        cpu_x86_load_seg_cache(env, sreg, selector, base, limit, flags);
+#endif
+    }
+    return 4;
+}
+
 static int cpu_gdb_write_register(CPUState *env, uint8_t *mem_buf, int n)
 {
     uint32_t tmp;
@@ -590,23 +615,12 @@ static int cpu_gdb_write_register(CPUState *env, uint8_t *mem_buf, int n)
             env->eflags = ldl_p(mem_buf);
             return 4;
 
-#if defined(CONFIG_USER_ONLY)
-#define LOAD_SEG(index, sreg)\
-            tmp = ldl_p(mem_buf);\
-            if (tmp != env->segs[sreg].selector)\
-                cpu_x86_load_seg(env, sreg, tmp);\
-            return 4
-#else
-/* FIXME: Honor segment registers.  Needs to avoid raising an exception
-   when the selector is invalid.  */
-#define LOAD_SEG(index, sreg) return 4
-#endif
-        case IDX_SEG_REGS:     LOAD_SEG(10, R_CS);
-        case IDX_SEG_REGS + 1: LOAD_SEG(11, R_SS);
-        case IDX_SEG_REGS + 2: LOAD_SEG(12, R_DS);
-        case IDX_SEG_REGS + 3: LOAD_SEG(13, R_ES);
-        case IDX_SEG_REGS + 4: LOAD_SEG(14, R_FS);
-        case IDX_SEG_REGS + 5: LOAD_SEG(15, R_GS);
+        case IDX_SEG_REGS:     return cpu_x86_gdb_load_seg(env, R_CS, mem_buf);
+        case IDX_SEG_REGS + 1: return cpu_x86_gdb_load_seg(env, R_SS, mem_buf);
+        case IDX_SEG_REGS + 2: return cpu_x86_gdb_load_seg(env, R_DS, mem_buf);
+        case IDX_SEG_REGS + 3: return cpu_x86_gdb_load_seg(env, R_ES, mem_buf);
+        case IDX_SEG_REGS + 4: return cpu_x86_gdb_load_seg(env, R_FS, mem_buf);
+        case IDX_SEG_REGS + 5: return cpu_x86_gdb_load_seg(env, R_GS, mem_buf);
 
         case IDX_FP_REGS + 8:
             env->fpuc = ldl_p(mem_buf);

target-i386/cpu.h

@@ -770,6 +770,10 @@ static inline void cpu_x86_load_seg_cache(CPUX86State *env,
     }
 }
 
+int cpu_x86_get_descr_debug(CPUX86State *env, unsigned int selector,
+                            target_ulong *base, unsigned int *limit,
+                            unsigned int *flags);
+
 /* wrapper, just in case memory mappings must be changed */
 static inline void cpu_x86_set_cpl(CPUX86State *s, int cpl)
 {

target-i386/helper.c

@@ -1782,6 +1782,36 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
     }
 }
 
+
+int cpu_x86_get_descr_debug(CPUX86State *env, unsigned int selector,
+                            target_ulong *base, unsigned int *limit,
+                            unsigned int *flags)
+{
+    SegmentCache *dt;
+    target_ulong ptr;
+    uint32_t e1, e2;
+    int index;
+
+    if (selector & 0x4)
+        dt = &env->ldt;
+    else
+        dt = &env->gdt;
+    index = selector & ~7;
+    ptr = dt->base + index;
+    if ((index + 7) > dt->limit
+        || cpu_memory_rw_debug(env, ptr, (uint8_t *)&e1, sizeof(e1), 0) != 0
+        || cpu_memory_rw_debug(env, ptr+4, (uint8_t *)&e2, sizeof(e2), 0) != 0)
+        return 0;
+
+    *base = ((e1 >> 16) | ((e2 & 0xff) << 16) | (e2 & 0xff000000));
+    *limit = (e1 & 0xffff) | (e2 & 0x000f0000);
+    if (e2 & DESC_G_MASK)
+        *limit = (*limit << 12) | 0xfff;
+    *flags = e2;
+
+    return 1;
+}
+
 CPUX86State *cpu_x86_init(const char *cpu_model)
 {
     CPUX86State *env;