Re: [Xen-devel] [PATCH] xen/timers: Document and improve the representation of the timer heap metadata

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.03.19 at 17:02, <andrew.cooper3@xxxxxxxxxx> wrote:
> --- a/xen/common/timer.c
> +++ b/xen/common/timer.c
> @@ -45,18 +45,27 @@ DEFINE_PER_CPU(s_time_t, timer_deadline);
>  
>  
> /****************************************************************************
>   * HEAP OPERATIONS.
> + *
> + * Slot 0 of the heap is never a valid timer pointer, and instead holds the
> + * heap metadata.
>   */
>  
> -#define GET_HEAP_SIZE(_h)     ((int)(((u16 *)(_h))[0]))
> -#define SET_HEAP_SIZE(_h,_v)  (((u16 *)(_h))[0] = (u16)(_v))
> +struct heap_metadata {
> +    uint16_t size, limit;
> +};
> +
> +static struct heap_metadata *heap_metadata(struct timer **heap)
> +{
> +    /* Check that our type-punning doesn't overflow into heap[1] */
> +    BUILD_BUG_ON(sizeof(struct heap_metadata) > sizeof(struct timer *));
>  
> -#define GET_HEAP_LIMIT(_h)    ((int)(((u16 *)(_h))[1]))
> -#define SET_HEAP_LIMIT(_h,_v) (((u16 *)(_h))[1] = (u16)(_v))
> +    return (struct heap_metadata *)&heap[0];
> +}
>  
>  /* Sink down element @pos of @heap. */
>  static void down_heap(struct timer **heap, int pos)
>  {
> -    int sz = GET_HEAP_SIZE(heap), nxt;
> +    int sz = heap_metadata(heap)->size, nxt;

While I realize that it'll alter generated code, I think this would be
a very good opportunity to convert various local variables to
unsigned int. But I won't insist, so with or without the extra change
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan



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