Re: [PATCH] tcg: Fix execution on Apple Silicon

[Roman Bolshakov <r.bolshakov@yadro.com>]

On Sat, Jan 02, 2021 at 03:21:02PM +0300, Roman Bolshakov wrote:
> Pages can't be both write and executable at the same time on Apple
> Silicon. macOS provides public API to switch write protection [1] for
> JIT applications, like TCG.
> 
> 1. https://developer.apple.com/documentation/apple_silicon/porting_just-in-time_compilers_to_apple_silicon
> 
> Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
> ---
> 
> Happy holidays, everyone.
> 
> This is somewhat similar to https://patchwork.kernel.org/project/qemu-devel/patch/20201108232425.1705-7-j@getutm.app/
> but I couldn't apply the series so I started from scratch.
> 
> The primary difference from the patch above is that public API is used.
> Other differences:
>   * TB pages are mostly kept write-locked except around tcg_qemu_tb_exec()
>   * x86_64 macOS doesn't use MAP_JIT and W^X switches
> 
> Regards,
> Roman
> 
>  accel/tcg/cpu-exec.c      | 10 ++++++++++
>  accel/tcg/translate-all.c | 26 ++++++++++++++++++++++++++
>  include/exec/exec-all.h   |  2 ++
>  tcg/tcg.c                 |  1 +
>  4 files changed, 39 insertions(+)
> 
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 8689c54499..0042fc9f2b 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -175,7 +175,9 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
>      }
>  #endif /* DEBUG_DISAS */
>  
> +    tb_write_lock();
>      ret = tcg_qemu_tb_exec(env, tb_ptr);
> +    tb_write_unlock();
>      cpu->can_do_io = 1;
>      last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
>      tb_exit = ret & TB_EXIT_MASK;
> @@ -220,9 +222,11 @@ static void cpu_exec_nocache(CPUState *cpu, int max_cycles,
>      cflags |= MIN(max_cycles, CF_COUNT_MASK);
>  
>      mmap_lock();
> +    tb_write_unlock();
>      tb = tb_gen_code(cpu, orig_tb->pc, orig_tb->cs_base,
>                       orig_tb->flags, cflags);
>      tb->orig_tb = orig_tb;
> +    tb_write_lock();
>      mmap_unlock();
>  
>      /* execute the generated code */
> @@ -268,7 +272,9 @@ void cpu_exec_step_atomic(CPUState *cpu)
>          tb = tb_lookup__cpu_state(cpu, &pc, &cs_base, &flags, cf_mask);
>          if (tb == NULL) {
>              mmap_lock();
> +            tb_write_unlock();
>              tb = tb_gen_code(cpu, pc, cs_base, flags, cflags);
> +            tb_write_lock();
>              mmap_unlock();
>          }
>  
> @@ -428,7 +434,9 @@ static inline TranslationBlock *tb_find(CPUState *cpu,
>      tb = tb_lookup__cpu_state(cpu, &pc, &cs_base, &flags, cf_mask);
>      if (tb == NULL) {
>          mmap_lock();
> +        tb_write_unlock();
>          tb = tb_gen_code(cpu, pc, cs_base, flags, cf_mask);
> +        tb_write_lock();
>          mmap_unlock();
>          /* We add the TB in the virtual pc hash table for the fast lookup */
>          qatomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);
> @@ -444,7 +452,9 @@ static inline TranslationBlock *tb_find(CPUState *cpu,
>  #endif
>      /* See if we can patch the calling TB. */
>      if (last_tb) {
> +        tb_write_unlock();
>          tb_add_jump(last_tb, tb_exit, tb);
> +        tb_write_lock();
>      }
>      return tb;
>  }
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index b7d50a73d4..1562076ffb 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -1072,6 +1072,9 @@ static inline void *alloc_code_gen_buffer(void)
>      size_t size = tcg_ctx->code_gen_buffer_size;
>      void *buf;
>  
> +#if defined(__APPLE__) && defined(__aarch64__)
> +    flags |= MAP_JIT;
> +#endif
>      buf = mmap(NULL, size, prot, flags, -1, 0);
>      if (buf == MAP_FAILED) {
>          return NULL;
> @@ -1485,7 +1488,9 @@ static void do_tb_phys_invalidate(TranslationBlock *tb, bool rm_from_page_list)
>  
>  static void tb_phys_invalidate__locked(TranslationBlock *tb)
>  {
> +    tb_write_unlock();
>      do_tb_phys_invalidate(tb, true);
> +    tb_write_lock();
>  }
>  
>  /* invalidate one TB
> @@ -2722,3 +2727,24 @@ void tcg_flush_softmmu_tlb(CPUState *cs)
>      tlb_flush(cs);
>  #endif
>  }
> +
> +#if defined(__APPLE__) && defined(__aarch64__)
> +static void tb_write_protect(bool locked)
> +{
> +    if (pthread_jit_write_protect_supported_np()){
> +        pthread_jit_write_protect_np(locked);
> +    }
> +}
> +#else
> +static void tb_write_protect(bool locked) {}
> +#endif
> +
> +void tb_write_lock(void)
> +{
> +    tb_write_protect(true);
> +}
> +
> +void tb_write_unlock(void)
> +{
> +    tb_write_protect(false);
> +}
> diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
> index fab573da06..962dca0975 100644
> --- a/include/exec/exec-all.h
> +++ b/include/exec/exec-all.h
> @@ -549,6 +549,8 @@ TranslationBlock *tb_htable_lookup(CPUState *cpu, target_ulong pc,
>                                     target_ulong cs_base, uint32_t flags,
>                                     uint32_t cf_mask);
>  void tb_set_jmp_target(TranslationBlock *tb, int n, uintptr_t addr);
> +void tb_write_lock(void);
> +void tb_write_unlock(void);
>  
>  /* GETPC is the true target of the return instruction that we'll execute.  */
>  #if defined(CONFIG_TCG_INTERPRETER)
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index 43c6cf8f52..303bb436bd 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -1065,6 +1065,7 @@ void tcg_prologue_init(TCGContext *s)
>      s->pool_labels = NULL;
>  #endif
>  
> +    tb_write_unlock();
>      /* Generate the prologue.  */
>      tcg_target_qemu_prologue(s);
>  
> -- 
> 2.29.2
> 

I've also noticed that Apple doesn't worry about sticking to particular
W^X mode:

https://bugs.webkit.org/attachment.cgi?id=402515&action=prettypatch

We might also drop lock/unlock symmetry from here. E.g. we can have two
functions that switch the mode (they might be moved to util/osdep.c):

  qemu_jit_write();
  qemu_jit_execute();

Then we use them just before writing or before executing like advised on
their documentation page.

-Roman