On Mon, Jan 14, 2019 at 04:17:49PM +0000, Frediano Ziglio wrote:
> The SpiceStat structure can be 20 or 24 bytes depending on alignment.
> Being a memory mapped structure potentially used with lockless access,
> it is not good to have it unaligned.
> The only tool that we know of that reads this memory mapped file
> (reds_stats) is able to detect if the structure is either 20 or 24
> bytes and act accordingly so changing this structure won't affect the
> behaviour (unless you have an old tool but as they are usually
> packaged together this is quite improbable).
> This will also help on Windows as in that system it is not possible
This is the main reason for the change, no? I would have started with
that.
Acked-by: Christophe Fergeau <cfergeau@xxxxxxxxxx>
> for reds_stats to implement the same discovery trick implemented on
> Linux. On Windows it is not possible to read the size of the file
> mapping (on Windows to implement shared memory you can use a file
> mapping not associated to a file).
> The structure will change layout only on 32-bit architectures which is
> not recommended nowadays (the 64-bit platforms that we support align
> 64-bit integers to 64 bits).
>
> Signed-off-by: Frediano Ziglio <fziglio@xxxxxxxxxx>
> ---
> spice/stats.h | 2 ++
> 1 file changed, 2 insertions(+)
>
> Changes since v3:
> - update commit message.
>
> Changes since v2:
> - typos and grammar.
>
> Changes since v1:
> - update commit message and comment.
>
> diff --git a/spice/stats.h b/spice/stats.h
> index d4c45ab..44c6482 100644
> --- a/spice/stats.h
> +++ b/spice/stats.h
> @@ -66,6 +66,8 @@ typedef struct SpiceStat {
> uint32_t generation;
> uint32_t num_of_nodes;
> uint32_t root_index;
> + /* to avoid holes in the structure on some platforms */
> + uint32_t padding;
> SpiceStatNode nodes[];
> } SpiceStat;
>
> --
> 2.20.1
>
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