Re: [Qemu-devel] [PATCH 03/23] memory: merge adjacent segments of a single memory region

[Anthony Liguori]

On 07/25/2011 09:02 AM, Avi Kivity wrote:
> Simple implementations of memory routers, for example the Cirrus VGA memory 
> banks
> or the 440FX PAM registers can generate adjacent memory regions which are 
> contiguous.
> Detect these and merge them; this saves kvm memory slots and shortens lookup 
> times.
>
> Signed-off-by: Avi Kivity<address@hidden>
> ---
>   memory.c |   22 ++++++++++++++++++++++
>   1 files changed, 22 insertions(+), 0 deletions(-)
>
> diff --git a/memory.c b/memory.c
> index a569666..339bea3 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -122,6 +122,27 @@ static void flatview_destroy(FlatView *view)
>       qemu_free(view->ranges);
>   }
>
> +/* Attempt to simplify a view by merging ajacent ranges */
> +static void flatview_simplify(FlatView *view)
> +{
> +    unsigned i;
> +    FlatRange *r1, *r2;
> +
> +    for (i = 0; i + 1<  view->nr; ++i) {
> +        r1 =&view->ranges[i];
> +        r2 =&view->ranges[i+1];
> +        if (addrrange_end(r1->addr) == r2->addr.start
> +&&  r1->mr == r2->mr
> +&&  r1->offset_in_region + r1->addr.size == r2->offset_in_region
> +&&  r1->dirty_log_mask == r2->dirty_log_mask) {
> +            r1->addr.size += r2->addr.size;
> +            memmove(r2, r2 + 1, (view->nr - (i + 2)) * sizeof(*r2));
> +            --view->nr;
> +            --i;
> +        }

The --i is pretty subtle.  Moving the index variable backwards in a 
conditional in a for loop is pretty evil :-)  I started writing up why 
this was wrong until I noticed that.

I think the following would be more straight forward:

i = 0;
while (i + 1 < view->nr) {
   int begin = i, end = i + 1;

   while (matches(&view->ranges[begin], &view->ranges[end])) {
      end++;
   }

   memmove(...)
}

Regards,

Anthony Liguori

> +    }
> +}
> +
>   /* Render a memory region into the global view.  Ranges in @view obscure
>    * ranges in @mr.
>    */
> @@ -209,6 +230,7 @@ static FlatView generate_memory_topology(MemoryRegion *mr)
>       flatview_init(&view);
>
>       render_memory_region(&view, mr, 0, addrrange_make(0, UINT64_MAX));
> +    flatview_simplify(&view);
>
>       return view;
>   }