Re: [Qemu-devel] [PATCH] tcg: Use the GDB JIT debugging interface.

[Jan Kiszka]

On 2011-09-30 01:46, Richard Henderson wrote:
> 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 <address@hidden>
> ---
>  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.

Cool.

Would it be possible to tag translated code as well? At TB or maybe even
input instruction level? Only in debugging mode of course.

> 
> 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)

Leading '_' are reserved for the system and tabu for the application (we
have some other violations, yes).

> +{
> +    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

#ifdef DEBUG_ELF_GENERATION or so?

> +    /* 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);

Jan

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