Re: [Qemu-devel] [PATCH 03/11] tci: Make direct jump patching thread-safe

["Alex Bennée" <alex.bennee@linaro.org>]

Sergey Fedorov <sergey.fedorov@linaro.org> writes:

> From: Sergey Fedorov <serge.fdrv@gmail.com>
>
> Ensure direct jump patching in TCI is atomic by:
>  * naturally aligning a location of direct jump address;
>  * using atomic_read()/atomic_set() to load/store the address.
>
> Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
> Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
> ---
>  include/exec/exec-all.h  | 2 +-
>  tcg/tci/tcg-target.inc.c | 2 ++
>  tci.c                    | 5 ++++-
>  3 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
> index 736209505a68..59709c9dd5c9 100644
> --- a/include/exec/exec-all.h
> +++ b/include/exec/exec-all.h
> @@ -302,7 +302,7 @@ void tb_phys_invalidate(TranslationBlock *tb, tb_page_addr_t page_addr);
>  static inline void tb_set_jmp_target1(uintptr_t jmp_addr, uintptr_t addr)
>  {
>      /* patch the branch destination */
> -    *(uint32_t *)jmp_addr = addr - (jmp_addr + 4);
> +    atomic_set((int32_t *)jmp_addr, addr - (jmp_addr + 4));
>      /* no need to flush icache explicitly */
>  }
>  #elif defined(_ARCH_PPC)
> diff --git a/tcg/tci/tcg-target.inc.c b/tcg/tci/tcg-target.inc.c
> index 4afe4d7a8d59..7e6180e62898 100644
> --- a/tcg/tci/tcg-target.inc.c
> +++ b/tcg/tci/tcg-target.inc.c
> @@ -556,6 +556,8 @@ static void tcg_out_op(TCGContext *s, TCGOpcode opc, const TCGArg *args,
>          if (s->tb_jmp_offset) {
>              /* Direct jump method. */
>              assert(args[0] < ARRAY_SIZE(s->tb_jmp_offset));
> +            /* Align for atomic patching and thread safety */
> +            s->code_ptr = (uint8_t *)(((uintptr_t)s->code_ptr + 3) &
> ~3);

Seeing this pattern is being used over and over again I wonder if we
should have some utility helper functions for this? Perhaps we should
steal the kernels ALIGN macros?

>              s->tb_jmp_offset[args[0]] = tcg_current_code_size(s);
>              tcg_out32(s, 0);
>          } else {
> diff --git a/tci.c b/tci.c
> index 82705fe77295..531f5ebf2c2e 100644
> --- a/tci.c
> +++ b/tci.c
> @@ -1089,7 +1089,10 @@ uintptr_t tcg_qemu_tb_exec(CPUArchState *env, uint8_t *tb_ptr)
>              goto exit;
>              break;
>          case INDEX_op_goto_tb:
> -            t0 = tci_read_i32(&tb_ptr);
> +            /* Jump address is aligned */
> +            tb_ptr = (uint8_t *)(((uintptr_t)tb_ptr + 3) & ~3);
> +            t0 = atomic_read((int32_t *)tb_ptr);
> +            tb_ptr += sizeof(int32_t);
>              tci_assert(tb_ptr == old_code_ptr + op_size);
>              tb_ptr += (int32_t)t0;
>              continue;


--
Alex Bennée