Re: [Qemu-devel] [PATCH 1/3] tcg/arm: fix branch target change during code retranslation

[Edgar E. Iglesias]

On Thu, Jan 06, 2011 at 10:54:32PM +0100, Aurelien Jarno wrote:
> QEMU uses code retranslation to restore the CPU state when an exception
> happens. For it to work the retranslation must not modify the generated
> code. This is what is currently implemented in ARM TCG.
> 
> However on CPU that don't have icache/dcache/memory synchronised like
> ARM, this requirement is stronger and code retranslation must not modify
> the generated code "atomically", as the cache line might be flushed
> at any moment (interrupt, exception, task switching), even if not
> triggered by QEMU. The probability for this to happen is very low, and
> depends on cache size and associativiy, machine load, interrupts, so the
> symptoms are might happen randomly.
> 
> This requirement is currently not followed in tcg/arm, for the
> load/store code, which basically has the following structure:
>   1) tlb access code is written
>   2) conditional fast path code is written
>   3) branch is written with a temporary target
>   4) slow path code is written
>   5) branch target is updated
> The cache lines corresponding to the retranslated code is not flushed
> after code retranslation as the generated code is supposed to be the
> same. However if the cache line corresponding to the branch instruction
> is flushed between step 3 and 5, and is not flushed again before the
> code is executed again, the branch target is wrong. In the guest, the
> symptoms are MMU page fault at a random addresses, which leads to
> kernel page fault or segmentation faults.
> 
> The patch fixes this issue by avoiding writing the branch target until
> it is known, that is by writing only the branch instruction first, and
> later only the offset.
> 
> This fixes booting linux guests on ARM hosts (tested: arm, i386, mips,
> mipsel, sh4, sparc).
> 
> Cc: Andrzej Zaborowski <address@hidden>
> Signed-off-by: Aurelien Jarno <address@hidden>

Good catch!

The patch looks good to me.

Acked-by: Edgar E. Iglesias <address@hidden>


> ---
>  tcg/arm/tcg-target.c |   28 ++++++++++++++++++++--------
>  1 files changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/tcg/arm/tcg-target.c b/tcg/arm/tcg-target.c
> index a3af5b2..9def2e5 100644
> --- a/tcg/arm/tcg-target.c
> +++ b/tcg/arm/tcg-target.c
> @@ -113,12 +113,25 @@ static const int tcg_target_call_oarg_regs[2] = {
>      TCG_REG_R0, TCG_REG_R1
>  };
>  
> +static inline void reloc_abs32(void *code_ptr, tcg_target_long target)
> +{
> +    *(uint32_t *) code_ptr = target;
> +}
> +
> +static inline void reloc_pc24(void *code_ptr, tcg_target_long target)
> +{
> +    uint32_t offset = ((target - ((tcg_target_long) code_ptr + 8)) >> 2);
> +
> +    *(uint32_t *) code_ptr = ((*(uint32_t *) code_ptr) & ~0xffffff)
> +                             | (offset & 0xffffff);
> +}
> +
>  static void patch_reloc(uint8_t *code_ptr, int type,
>                  tcg_target_long value, tcg_target_long addend)
>  {
>      switch (type) {
>      case R_ARM_ABS32:
> -        *(uint32_t *) code_ptr = value;
> +        reloc_abs32(code_ptr, value);
>          break;
>  
>      case R_ARM_CALL:
> @@ -127,8 +140,7 @@ static void patch_reloc(uint8_t *code_ptr, int type,
>          tcg_abort();
>  
>      case R_ARM_PC24:
> -        *(uint32_t *) code_ptr = ((*(uint32_t *) code_ptr) & 0xff000000) |
> -                (((value - ((tcg_target_long) code_ptr + 8)) >> 2) & 
> 0xffffff);
> +        reloc_pc24(code_ptr, value);
>          break;
>      }
>  }
> @@ -1031,7 +1043,7 @@ static inline void tcg_out_qemu_ld(TCGContext *s, const 
> TCGArg *args, int opc)
>      }
>  
>      label_ptr = (void *) s->code_ptr;
> -    tcg_out_b(s, COND_EQ, 8);
> +    tcg_out_b_noaddr(s, COND_EQ);
>  
>      /* TODO: move this code to where the constants pool will be */
>      if (addr_reg != TCG_REG_R0) {
> @@ -1076,7 +1088,7 @@ static inline void tcg_out_qemu_ld(TCGContext *s, const 
> TCGArg *args, int opc)
>          break;
>      }
>  
> -    *label_ptr += ((void *) s->code_ptr - (void *) label_ptr - 8) >> 2;
> +    reloc_pc24(label_ptr, (tcg_target_long)s->code_ptr);
>  #else /* !CONFIG_SOFTMMU */
>      if (GUEST_BASE) {
>          uint32_t offset = GUEST_BASE;
> @@ -1245,7 +1257,7 @@ static inline void tcg_out_qemu_st(TCGContext *s, const 
> TCGArg *args, int opc)
>      }
>  
>      label_ptr = (void *) s->code_ptr;
> -    tcg_out_b(s, COND_EQ, 8);
> +    tcg_out_b_noaddr(s, COND_EQ);
>  
>      /* TODO: move this code to where the constants pool will be */
>      tcg_out_dat_reg(s, COND_AL, ARITH_MOV,
> @@ -1317,7 +1329,7 @@ static inline void tcg_out_qemu_st(TCGContext *s, const 
> TCGArg *args, int opc)
>      if (opc == 3)
>          tcg_out_dat_imm(s, COND_AL, ARITH_ADD, TCG_REG_R13, TCG_REG_R13, 
> 0x10);
>  
> -    *label_ptr += ((void *) s->code_ptr - (void *) label_ptr - 8) >> 2;
> +    reloc_pc24(label_ptr, (tcg_target_long)s->code_ptr);
>  #else /* !CONFIG_SOFTMMU */
>      if (GUEST_BASE) {
>          uint32_t offset = GUEST_BASE;
> @@ -1399,7 +1411,7 @@ static inline void tcg_out_op(TCGContext *s, TCGOpcode 
> opc,
>              /* Direct jump method */
>  #if defined(USE_DIRECT_JUMP)
>              s->tb_jmp_offset[args[0]] = s->code_ptr - s->code_buf;
> -            tcg_out_b(s, COND_AL, 8);
> +            tcg_out_b_noaddr(s, COND_AL);
>  #else
>              tcg_out_ld32_12(s, COND_AL, TCG_REG_PC, TCG_REG_PC, -4);
>              s->tb_jmp_offset[args[0]] = s->code_ptr - s->code_buf;
> -- 
> 1.7.2.3
> 
>