[PATCH] tcg: Use the GDB JIT debugging interface.

[Richard Henderson <rth at twiddle dot net>]

This allows us to generate unwind info for the dynamicly generated
code in the code_gen_buffer.  Only i386 is converted at this point.

Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 elf.h                 |    1 +
 exec.c                |    9 +++
 tcg/i386/tcg-target.c |  125 +++++++++++++++++++++++++++++++---
 tcg/tcg.c             |  185 +++++++++++++++++++++++++++++++++++++++++++++++++
 tcg/tcg.h             |    2 +
 5 files changed, 313 insertions(+), 9 deletions(-)


It's somewhat unfortunate, but the GDB interface requires that we
build a contiguous ELF image.  This means we can't place the ELF
header that we generate any place except the beginning of the
code_gen_buffer.  While tedious, this isn't terribly difficult.

With this patch, I now get:

Breakpoint 1, __ldb_mmu (addr=1001716, mmu_idx=0)
    at /home/rth/work/qemu/qemu/softmmu_template.h:86
86	{
(gdb) where
#0  __ldb_mmu (addr=1001716, mmu_idx=0)
    at /home/rth/work/qemu/qemu/softmmu_template.h:86
#1  0x0000000040000afc in ?? ()
#2  0x000000000053e85c in cpu_x86_exec (env=0x1208d80)
    at /home/rth/work/qemu/qemu/cpu-exec.c:565
#3  0x0000000000542932 in tcg_cpu_exec (env=0x1208d80)
    at /home/rth/work/qemu/qemu/cpus.c:913
#4  cpu_exec_all () at /home/rth/work/qemu/qemu/cpus.c:949
#5  0x0000000000542ad5 in qemu_tcg_cpu_thread_fn (arg=<optimized out>)
    at /home/rth/work/qemu/qemu/cpus.c:688
#6  0x00000033f1a07b31 in start_thread () from /lib64/libpthread.so.0
#7  0x00000033f16dfd2d in clone () from /lib64/libc.so.6

which is nicely correct.

Comments from the gdb folk are welcome.


r~



diff --git a/elf.h b/elf.h
index 2e05d34..1e56a8c 100644
--- a/elf.h
+++ b/elf.h
@@ -216,6 +216,7 @@ typedef int64_t  Elf64_Sxword;
 
 #define ELF_ST_BIND(x)		((x) >> 4)
 #define ELF_ST_TYPE(x)		(((unsigned int) x) & 0xf)
+#define ELF_ST_INFO(bind,type)	(((bind) << 4) | (type))
 #define ELF32_ST_BIND(x)	ELF_ST_BIND(x)
 #define ELF32_ST_TYPE(x)	ELF_ST_TYPE(x)
 #define ELF64_ST_BIND(x)	ELF_ST_BIND(x)
diff --git a/exec.c b/exec.c
index 1e6f732..f6c07d5 100644
--- a/exec.c
+++ b/exec.c
@@ -464,6 +464,8 @@ static uint8_t static_code_gen_buffer[DEFAULT_CODE_GEN_BUFFER_SIZE]
 
 static void code_gen_alloc(unsigned long tb_size)
 {
+    size_t grab;
+
 #ifdef USE_STATIC_CODE_GEN_BUFFER
     code_gen_buffer = static_code_gen_buffer;
     code_gen_buffer_size = DEFAULT_CODE_GEN_BUFFER_SIZE;
@@ -558,6 +560,13 @@ static void code_gen_alloc(unsigned long tb_size)
     map_exec(code_gen_buffer, code_gen_buffer_size);
 #endif
 #endif /* !USE_STATIC_CODE_GEN_BUFFER */
+
+    /* Give GDB unwind info for the code we generate.  This grabs a small
+       amount of space from the front of the buffer.  Account for it.  */
+    grab = tcg_register_jit(code_gen_buffer, code_gen_buffer_size);
+    code_gen_buffer += grab;
+    code_gen_buffer_size -= grab;
+
     map_exec(code_gen_prologue, sizeof(code_gen_prologue));
     code_gen_buffer_max_size = code_gen_buffer_size -
         (TCG_MAX_OP_SIZE * OPC_BUF_SIZE);
diff --git a/tcg/i386/tcg-target.c b/tcg/i386/tcg-target.c
index 281f87d..462f455 100644
--- a/tcg/i386/tcg-target.c
+++ b/tcg/i386/tcg-target.c
@@ -1913,22 +1913,29 @@ static int tcg_target_callee_save_regs[] = {
 #endif
 };
 
+/* Compute frame size via macros, to share between tcg_target_qemu_prologue
+   and tcg_register_jit.  */
+
+#define PUSH_SIZE \
+    ((1 + ARRAY_SIZE(tcg_target_callee_save_regs)) \
+     * (TCG_TARGET_REG_BITS / 8))
+
+#define FRAME_SIZE \
+    ((PUSH_SIZE \
+      + TCG_STATIC_CALL_ARGS_SIZE \
+      + CPU_TEMP_BUF_NLONGS * sizeof(long) \
+      + TCG_TARGET_STACK_ALIGN - 1) \
+     & ~(TCG_TARGET_STACK_ALIGN - 1))
+
 /* Generate global QEMU prologue and epilogue code */
 static void tcg_target_qemu_prologue(TCGContext *s)
 {
-    int i, frame_size, push_size, stack_addend;
+    int i, stack_addend;
 
     /* TB prologue */
 
     /* Reserve some stack space, also for TCG temps.  */
-    push_size = 1 + ARRAY_SIZE(tcg_target_callee_save_regs);
-    push_size *= TCG_TARGET_REG_BITS / 8;
-
-    frame_size = push_size + TCG_STATIC_CALL_ARGS_SIZE +
-        CPU_TEMP_BUF_NLONGS * sizeof(long);
-    frame_size = (frame_size + TCG_TARGET_STACK_ALIGN - 1) &
-        ~(TCG_TARGET_STACK_ALIGN - 1);
-    stack_addend = frame_size - push_size;
+    stack_addend = FRAME_SIZE - PUSH_SIZE;
     tcg_set_frame(s, TCG_REG_CALL_STACK, TCG_STATIC_CALL_ARGS_SIZE,
                   CPU_TEMP_BUF_NLONGS * sizeof(long));
 
@@ -1988,3 +1995,103 @@ static void tcg_target_init(TCGContext *s)
 
     tcg_add_target_add_op_defs(x86_op_defs);
 }
+
+extern char debug_frame[];
+extern char debug_frame_end[];
+extern void *debug_frame_code_start;
+extern size_t debug_frame_code_len;
+extern char debug_frame_frame_size[];
+
+#if TCG_TARGET_REG_BITS == 64
+#define ELF_HOST_MACHINE EM_X86_64
+asm(".data\n"
+"	.align 8\n"
+"debug_frame:\n"
+"	.long	99f-0f\n"	/* Length of CIE */
+"0:	.long	-1\n"		/* CIE identifier */
+"	.byte	1\n"		/* CIE version */
+"	.byte	0\n"		/* CIE augmentation (none) */
+"	.byte	1\n"		/* CIE code alignment factor */
+"	.byte	0x78\n"		/* CIE data alignment factor (sleb128 -8) */
+"	.byte	16\n"		/* CIE return address column */
+"	.align 8\n"
+"99:\n"				/* End of CIE */
+"	.long	99f-0f\n"	/* Length of FDE */
+"0:	.long	0\n"		/* FDE CIE offset (start of section) */
+"debug_frame_code_start:\n"
+"	.quad	0\n"		/* FDE start (to be filled in) */
+"debug_frame_code_len:\n"
+"	.quad	0\n"		/* FDE length (to be filled in) */
+"	.byte	12\n"		/* DW_CFA_def_cfa */
+"	.byte	7\n"		/*   %rsp */
+"debug_frame_frame_size:\n"
+"	.byte	0, 0\n"		/*   frame_size (to be filled in) */
+"	.byte	0x90, 1\n"	/* DW_CFA_offset, %rip, -8 */
+/* The following ordering must match tcg_target_callee_save_regs.  */
+"	.byte	0x86, 2\n"	/* DW_CFA_offset, %rbp, -16 */
+"	.byte	0x83, 3\n"	/* DW_CFA_offset, %rbx, -24 */
+"	.byte	0x8c, 4\n"	/* DW_CFA_offset, %r12, -32 */
+"	.byte	0x8d, 5\n"	/* DW_CFA_offset, %r13, -40 */
+"	.byte	0x8e, 6\n"	/* DW_CFA_offset, %r14, -48 */
+"	.byte	0x8f, 7\n"	/* DW_CFA_offset, %r15, -56 */
+"	.align	8\n"
+"99:\n"				/* End of FDE */
+"debug_frame_end:\n"
+".previous"
+);
+#else
+#define ELF_HOST_MACHINE EM_386
+asm(".data\n"
+"	.align 4\n"
+"debug_frame:\n"
+"	.long	99f-0f\n"	/* Length of CIE */
+"0:	.long	-1\n"		/* CIE identifier */
+"	.byte	1\n"		/* CIE version */
+"	.byte	0\n"		/* CIE augmentation (none) */
+"	.byte	1\n"		/* CIE code alignment factor */
+"	.byte	0x7c\n"		/* CIE data alignment factor (sleb128 -4) */
+"	.byte	8\n"		/* CIE return address column */
+"	.align 4\n"
+"99:\n"				/* End of CIE */
+"	.long	99f-0f\n"	/* Length of FDE */
+"0:	.long	0\n"		/* FDE CIE offset (start of section) */
+"debug_frame_code_start:\n"
+"	.long	0\n"		/* FDE start (to be filled in) */
+"debug_frame_code_len:\n"
+"	.long	0\n"		/* FDE length (to be filled in) */
+"	.byte	12\n"		/* DW_CFA_def_cfa */
+"	.byte	4\n"		/*   %rsp */
+"debug_frame_frame_size:\n"
+"	.byte	0, 0\n"		/*   frame_size (to be filled in) */
+"	.byte	0x88, 1\n"	/* DW_CFA_offset, %eip, -4 */
+/* The following ordering must match tcg_target_callee_save_regs.  */
+"	.byte	0x85, 2\n"	/* DW_CFA_offset, %ebp, -8 */
+"	.byte	0x83, 3\n"	/* DW_CFA_offset, %ebx, -12 */
+"	.byte	0x86, 4\n"	/* DW_CFA_offset, %esi, -16 */
+"	.byte	0x87, 5\n"	/* DW_CFA_offset, %edi, -20 */
+"	.align	4\n"
+"99:\n"				/* End of FDE */
+"debug_frame_end:\n"
+".previous"
+);
+#endif
+
+size_t tcg_register_jit(void *buf, size_t buf_size)
+{
+    const unsigned int frame_size = FRAME_SIZE;
+    unsigned int f_lo, f_hi;
+
+    /* ??? These could be filled in generically via reading the debug data.  */
+    debug_frame_code_start = buf;
+    debug_frame_code_len = buf_size;
+
+    /* ??? We're expecting a 2 byte uleb128 encoded value.  */
+    f_lo = (frame_size & 0x7f) | 0x80;
+    f_hi = frame_size >> 7;
+    assert(frame_size >> 14 == 0);
+    debug_frame_frame_size[0] = f_lo;
+    debug_frame_frame_size[1] = f_hi;
+
+    return tcg_register_jit_int(buf, buf_size, debug_frame,
+                                debug_frame_end - debug_frame);
+}
diff --git a/tcg/tcg.c b/tcg/tcg.c
index 411f971..63d81f9 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -57,6 +57,18 @@
 #include "cpu.h"
 
 #include "tcg-op.h"
+
+#if TCG_TARGET_REG_BITS == 64
+# define ELF_CLASS  ELFCLASS64
+#else
+# define ELF_CLASS  ELFCLASS32
+#endif
+#ifdef HOST_WORDS_BIGENDIAN
+# define ELF_DATA   ELFDATA2MSB
+#else
+# define ELF_DATA   ELFDATA2LSB
+#endif
+
 #include "elf.h"
 
 #if defined(CONFIG_USE_GUEST_BASE) && !defined(TCG_TARGET_HAS_GUEST_BASE)
@@ -68,6 +80,9 @@ static void tcg_target_qemu_prologue(TCGContext *s);
 static void patch_reloc(uint8_t *code_ptr, int type, 
                         tcg_target_long value, tcg_target_long addend);
 
+static size_t tcg_register_jit_int(void *buf, size_t size,
+                                   void *debug_frame, size_t debug_frame_size);
+
 TCGOpDef tcg_op_defs[] = {
 #define DEF(s, oargs, iargs, cargs, flags) { #s, oargs, iargs, cargs, iargs + oargs + cargs, flags },
 #include "tcg-opc.h"
@@ -2233,3 +2248,173 @@ void tcg_dump_info(FILE *f, fprintf_function cpu_fprintf)
     cpu_fprintf(f, "[TCG profiler not compiled]\n");
 }
 #endif
+
+#ifdef ELF_HOST_MACHINE
+/* The backend should define ELF_HOST_MACHINE to indicate both what value to
+   put into the ELF image and to indicate support for the feature.  */
+
+/* Begin GDB interface.  The following must match the docs.  */
+typedef enum
+{
+    JIT_NOACTION = 0,
+    JIT_REGISTER_FN,
+    JIT_UNREGISTER_FN
+} jit_actions_t;
+
+struct jit_code_entry {
+    struct jit_code_entry *next_entry;
+    struct jit_code_entry *prev_entry;
+    const char *symfile_addr;
+    uint64_t symfile_size;
+};
+
+struct jit_descriptor {
+    uint32_t version;
+    uint32_t action_flag;
+    struct jit_code_entry *relevant_entry;
+    struct jit_code_entry *first_entry;
+};
+
+void __jit_debug_register_code(void);
+void __attribute__((noinline)) __jit_debug_register_code(void)
+{
+    asm("");
+}
+
+/* Must statically initialize the version, because GDB may check
+   the version before we can set it.  */
+struct jit_descriptor __jit_debug_descriptor = { 1, 0, 0, 0 };
+
+/* End GDB interface.  */
+
+static int find_string(const char *strtab, const char *str)
+{
+    const char *p = strtab + 1;
+
+    while (1) {
+        if (strcmp(p, str) == 0) {
+            return p - strtab;
+        }
+        p += strlen(p) + 1;
+    }
+}
+
+static size_t tcg_register_jit_int(void *buf, size_t buf_size,
+                                   void *debug_frame, size_t debug_frame_size)
+{
+    static const char strings[64] =
+        "\0"
+        ".text\0"
+        ".debug_frame\0"
+        ".symtab\0"
+        ".strtab\0"
+        "code_gen_buffer";
+
+    struct ElfImage {
+        ElfW(Ehdr) ehdr;
+        ElfW(Phdr) phdr;
+        ElfW(Shdr) shdr[5];
+        ElfW(Sym)  sym[1];
+        char       str[64];
+    };
+
+    /* We only need a single jit entry; statically allocate it.  */
+    static struct jit_code_entry one_entry;
+
+    struct ElfImage *img = buf;
+    size_t grab = sizeof(*img) + debug_frame_size;
+
+    img->ehdr.e_ident[EI_MAG0] = ELFMAG0;
+    img->ehdr.e_ident[EI_MAG1] = ELFMAG1;
+    img->ehdr.e_ident[EI_MAG2] = ELFMAG2;
+    img->ehdr.e_ident[EI_MAG3] = ELFMAG3;
+    img->ehdr.e_ident[EI_CLASS] = ELF_CLASS;
+    img->ehdr.e_ident[EI_DATA] = ELF_DATA;
+    img->ehdr.e_ident[EI_VERSION] = EV_CURRENT;
+    img->ehdr.e_type = ET_EXEC;
+    img->ehdr.e_machine = ELF_HOST_MACHINE;
+    img->ehdr.e_version = EV_CURRENT;
+    img->ehdr.e_phoff = offsetof(struct ElfImage, phdr);
+    img->ehdr.e_shoff = offsetof(struct ElfImage, shdr);
+    img->ehdr.e_ehsize = sizeof(ElfW(Shdr));
+    img->ehdr.e_phentsize = sizeof(ElfW(Phdr));
+    img->ehdr.e_phnum = 1;
+    img->ehdr.e_shentsize = sizeof(img->shdr[0]);
+    img->ehdr.e_shnum = ARRAY_SIZE(img->shdr);
+    img->ehdr.e_shstrndx = ARRAY_SIZE(img->shdr) - 1;
+
+    img->phdr.p_type = PT_LOAD;
+    img->phdr.p_offset = (char *)buf - (char *)img;
+    img->phdr.p_vaddr = (ElfW(Addr))buf;
+    img->phdr.p_paddr = img->phdr.p_vaddr;
+    img->phdr.p_filesz = buf_size;
+    img->phdr.p_memsz = buf_size;
+    img->phdr.p_flags = PF_X;
+
+    memcpy(img->str, strings, sizeof(img->str));
+
+    img->shdr[0].sh_type = SHT_NULL;
+
+    img->shdr[1].sh_name = find_string(img->str, ".text");
+    img->shdr[1].sh_type = SHT_PROGBITS;
+    img->shdr[1].sh_flags = SHF_EXECINSTR | SHF_ALLOC;
+    img->shdr[1].sh_addr = (ElfW(Addr))buf + grab;
+    img->shdr[1].sh_offset = grab;
+    img->shdr[1].sh_size = buf_size - grab;
+
+    img->shdr[2].sh_name = find_string(img->str, ".debug_frame");
+    img->shdr[2].sh_type = SHT_PROGBITS;
+    img->shdr[2].sh_offset = sizeof(*img);
+    img->shdr[2].sh_size = debug_frame_size;
+    memcpy(img + 1, debug_frame, debug_frame_size);
+
+    img->shdr[3].sh_name = find_string(img->str, ".symtab");
+    img->shdr[3].sh_type = SHT_SYMTAB;
+    img->shdr[3].sh_offset = offsetof(struct ElfImage, sym);
+    img->shdr[3].sh_size = sizeof(img->sym);
+    img->shdr[3].sh_info = ARRAY_SIZE(img->sym);
+    img->shdr[3].sh_link = img->ehdr.e_shstrndx;
+    img->shdr[3].sh_entsize = sizeof(ElfW(Sym));
+
+    img->shdr[4].sh_name = find_string(img->str, ".strtab");
+    img->shdr[4].sh_type = SHT_STRTAB;
+    img->shdr[4].sh_offset = offsetof(struct ElfImage, str);
+    img->shdr[4].sh_size = sizeof(img->str);
+
+    img->sym[0].st_name = find_string(img->str, "code_gen_buffer");
+    img->sym[0].st_info = ELF_ST_INFO(STB_GLOBAL, STT_FUNC);
+    img->sym[0].st_shndx = 1;
+    img->sym[0].st_value = (ElfW(Addr))buf + grab;
+    img->sym[0].st_size = buf_size - grab;
+
+#if 0
+    /* Enable this block to be able to debug the ELF image file creation.
+       One can use readelf, objdump, or other inspection utilities.  */
+    {
+        FILE *f = fopen("/tmp/qemu.jit", "w+b");
+        if (f) {
+            if (fwrite(buf, buf_size, 1, f) != buf_size) {
+                /* Avoid stupid unused return value warning for fwrite.  */
+            }
+            fclose(f);
+        }
+    }
+#endif
+
+    one_entry.symfile_addr = buf;
+    one_entry.symfile_size = buf_size;
+
+    __jit_debug_descriptor.action_flag = JIT_REGISTER_FN;
+    __jit_debug_descriptor.relevant_entry = &one_entry;
+    __jit_debug_descriptor.first_entry = &one_entry;
+    __jit_debug_register_code();
+
+    return grab;
+}
+#else
+/* No support for the feature.  Provide the entry point expected by exec.c.  */
+size_t tcg_register_jit(void *buf, size_t buf_size)
+{
+    return 0;
+}
+#endif /* ELF_HOST_MACHINE */
diff --git a/tcg/tcg.h b/tcg/tcg.h
index dc5e9c9..3d2e5d0 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -574,3 +574,5 @@ extern uint8_t code_gen_prologue[];
 #define tcg_qemu_tb_exec(env, tb_ptr)                                    \
     ((long REGPARM (*)(void *, void *))code_gen_prologue)(env, tb_ptr)
 #endif
+
+size_t tcg_register_jit(void *, size_t);
-- 
1.7.6.2