Re: [RFC PATCH 3/3] fuzz: Add callbacks for dma-access functions

[Stefan Hajnoczi]

On Thu, Jun 11, 2020 at 01:56:51AM -0400, Alexander Bulekov wrote:
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> ---
>  exec.c                                | 17 ++++++++++++++++-
>  include/exec/memory.h                 |  8 ++++++++
>  include/exec/memory_ldst_cached.inc.h |  9 +++++++++
>  include/sysemu/dma.h                  |  5 ++++-
>  memory_ldst.inc.c                     | 12 ++++++++++++
>  5 files changed, 49 insertions(+), 2 deletions(-)

Please rename dma_read_cb() to fuzz_dma_read_cb() so the purpose of the
function is clear.

The ifdefs can be avoided by defining an empty function when CONFIG_FUZZ
is undefined. In a header file:

  #ifdef CONFIG_FUZZ
  void fuzz_dma_read_cb(size_t addr, size_t len);
  #else
  static inline void fuzz_dma_read_cb(size_t addr, size_t len)
  {
      /* Do nothing */
  }
  #endif

Now the compiler should eliminate the deadcode:

  #ifdef CONFIG_FUZZ
  if (as->root == get_system_memory()) {
      dma_read_cb(addr, len);
  }
  #endif

becomes:

  if (as->root == get_system_memory()) {
      fuzz_dma_read_cb(addr, len);
  }

Hopefully gcc and clang will eliminate this and emit no instructions
when CONFIG_FUZZ is undefined. If not, you can simply pass in 'as' and
'is_write' too:

  void fuzz_dma_read_cb(AddressSpace *as, bool is_write, size_t addr, size_t 
len)

This way the conditional is moved inside fuzz_dma_read_cb() and deadcode
elimination becomes trivial for the compiler:

  fuzz_read_cb(as, is_write, addr, len);

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