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Re: [libvirt] [PATCH 4/6] remote-driver: Add domainMemoryStats support



On Fri, Dec 11, 2009 at 03:26:15PM -0500, Adam Litke wrote:
> Use a dynamically sized xdr_array to pass memory stats on the wire.  This
> supports the addition of future memory stats and reduces the message size since
> only supported statistics are returned.
> +    /* Allocate return buffer.  We need 2 slots per stat: the tag and value. */
> +    if (VIR_ALLOC_N(ret->stats.stats_val, nr_stats * 2) < 0) {
> +        VIR_FREE(stats);
> +        remoteDispatchOOMError(rerr);
> +        return -1;
> +    }
> +
> +    /* Copy the stats into the xdr return structure */
> +    for (i = 0; i < nr_stats; i++) {
> +        ret->stats.stats_val[2 * i] = (uint64_t) stats[i].tag;
> +        ret->stats.stats_val[2 * i + 1] = stats[i].val;
> +    }
> +    ret->stats.stats_len = nr_stats * 2;

This is a slightly wierd way to encode the data on the wire. The
wire format ought to follow the public API struct format, rather
than packing both values into a single array.

> +struct remote_domain_memory_stats_args {
> +        remote_nonnull_domain dom;
> +        u_int maxStats;
> +};
> +typedef struct remote_domain_memory_stats_args remote_domain_memory_stats_args;
> +
> +struct remote_domain_memory_stats_ret {
> +        struct {
> +                u_int stats_len;
> +                uint64_t *stats_val;
> +        } stats;
> +};

XDR has a native array type which automatically tracks length. Also
for portability to OS-X/Solaris we avoid uint64_t and use hyper
instead for 64bit data.

The return value structure should thus look like this:

  const REMOTE_DOMAIN_MEMORY_STATS_MAX = 1024;

  struct remote_domain_memory_stat {
      int tag;
      unsigned hyper val;
  };

  struct remote_domain_memory_stats_ret {
      remote_domain_memory_stat<REMOTE_DOMAIN_MEMORY_STATS_MAX>;
  };


NB, the MAX constant there should not be fixed to the public API
max. This is just a wire protocol decoder limit to avoid DOS on
large arrays.

Regards,
Daniel
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