Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes:
On Sun, 23 Aug 2020 17:08:30 +0300
Shay Agroskin <shayagr@xxxxxxxxxx> wrote:
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 3814fb631d52..42f439f9fcda 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -72,7 +72,8 @@ struct xdp_buff {
> void *data_hard_start;
> struct xdp_rxq_info *rxq;
> struct xdp_txq_info *txq;
> - u32 frame_sz; /* frame size to deduce
> data_hard_end/reserved tailroom*/
> + u32 frame_sz:31; /* frame size to deduce
> data_hard_end/reserved tailroom*/
> + u32 mb:1; /* xdp non-linear buffer */
> };
>
> /* Reserve memory area at end-of data area.
> @@ -96,7 +97,8 @@ struct xdp_frame {
> u16 len;
> u16 headroom;
> u32 metasize:8;
> - u32 frame_sz:24;
> + u32 frame_sz:23;
> + u32 mb:1; /* xdp non-linear frame */
Although this issue wasn't introduced with this patch, why not
make frame_sz field to be the same size in xdp_buff and
xdp_frame
?
This is all about struct layout and saving memory size, due to
cacheline access. Please read up on this and use the tool pahole
to
inspect the struct memory layout.
I actually meant reducing the size of frame_sz in xdp_buff
(without changing xdp_frame so that it still fits 64 byte cache
line). Reducing a field size shouldn't affect cache alignment as
far as I can see.
Doesn't matter all that much to me, I simply find it a better
practice that the same field would have same size in different
structs.
Shay
