Re: [Qemu-devel] [PATCH 2/4] exec: add wrapper for host pointer access

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Michael S. Tsirkin (mst@redhat.com) wrote:
> host pointer accesses force pointer math, let's
> add a wrapper to make them safer.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/exec/cpu-all.h |  5 +++++
>  exec.c                 | 10 +++++-----
>  2 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
> index c085804..9d8d408 100644
> --- a/include/exec/cpu-all.h
> +++ b/include/exec/cpu-all.h
> @@ -313,6 +313,11 @@ typedef struct RAMBlock {
>      int fd;
>  } RAMBlock;
>  
> +static inline void *ramblock_ptr(RAMBlock *block, ram_addr_t offset)
> +{
> +    return (char *)block->host + offset;
> +}

I'm a bit surprised you don't need to pass a length to this to be able
to tell how much you can access.

>  typedef struct RAMList {
>      QemuMutex mutex;
>      /* Protected by the iothread lock.  */
> diff --git a/exec.c b/exec.c
> index ad5cf12..9648669 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -840,7 +840,7 @@ static void tlb_reset_dirty_range_all(ram_addr_t start, ram_addr_t length)
>  
>      block = qemu_get_ram_block(start);
>      assert(block == qemu_get_ram_block(end - 1));
> -    start1 = (uintptr_t)block->host + (start - block->offset);
> +    start1 = (uintptr_t)ramblock_ptr(block, start - block->offset);
>      cpu_tlb_reset_dirty_all(start1, length);
>  }
>  
> @@ -1500,7 +1500,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
>      QTAILQ_FOREACH(block, &ram_list.blocks, next) {
>          offset = addr - block->offset;
>          if (offset < block->length) {
> -            vaddr = block->host + offset;
> +            vaddr = ramblock_ptr(block, offset);
>              if (block->flags & RAM_PREALLOC) {
>                  ;
>              } else if (xen_enabled()) {
> @@ -1551,7 +1551,7 @@ void *qemu_get_ram_block_host_ptr(ram_addr_t addr)
>  {
>      RAMBlock *block = qemu_get_ram_block(addr);
>  
> -    return block->host;
> +    return ramblock_ptr(block, 0);
>  }
>  
>  /* Return a host pointer to ram allocated with qemu_ram_alloc.
> @@ -1578,7 +1578,7 @@ void *qemu_get_ram_ptr(ram_addr_t addr)
>                  xen_map_cache(block->offset, block->length, 1);
>          }
>      }
> -    return block->host + (addr - block->offset);
> +    return ramblock_ptr(block, addr - block->offset);
>  }

which then makes me wonder if all the uses of this are safe near the
end of the block.

>  /* Return a host pointer to guest's ram. Similar to qemu_get_ram_ptr
> @@ -1597,7 +1597,7 @@ static void *qemu_ram_ptr_length(ram_addr_t addr, hwaddr *size)
>              if (addr - block->offset < block->length) {
>                  if (addr - block->offset + *size > block->length)
>                      *size = block->length - addr + block->offset;
> -                return block->host + (addr - block->offset);
> +                return ramblock_ptr(block, addr - block->offset);
>              }

but then this sounds like it's going to have partial duplication, it already looks
like it's only going to succeed if it finds itself a block that the access fits
in.


Dave

>          }
>  
> -- 
> MST
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK