Often times the compilers are not able to expand two consecutive 32-bit
writes into one 64-bit on the corresponding architectures. This applies
to xdp_init_buff() called for every received frame (or at least once
per each 64 frames when the frag size is fixed).
Move the not-so-pretty hack from libeth_xdp straight to xdp_init_buff(),
but using a proper union around ::frame_sz and ::flags.
The optimization is limited to LE architectures due to the structure
layout.
One simple example from idpf with the XDP series applied (Clang 22-git,
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE => -O2):
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-27 (-27)
Function old new delta
idpf_vport_splitq_napi_poll 5076 5049 -27
The perf difference with XDP_DROP is around +0.8-1% which I see as more
than satisfying.
Suggested-by: Simon Horman <horms@xxxxxxxxxx>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
---
include/net/libeth/xdp.h | 11 +----------
include/net/xdp.h | 28 +++++++++++++++++++++++++---
2 files changed, 26 insertions(+), 13 deletions(-)
diff --git a/include/net/libeth/xdp.h b/include/net/libeth/xdp.h
index f4880b50e804..bc3507edd589 100644
--- a/include/net/libeth/xdp.h
+++ b/include/net/libeth/xdp.h
@@ -1274,7 +1274,6 @@ bool libeth_xdp_buff_add_frag(struct libeth_xdp_buff *xdp,
* Internal, use libeth_xdp_process_buff() instead. Initializes XDP buffer
* head with the Rx buffer data: data pointer, length, headroom, and
* truesize/tailroom. Zeroes the flags.
- * Uses faster single u64 write instead of per-field access.
*/
static inline void libeth_xdp_prepare_buff(struct libeth_xdp_buff *xdp,
const struct libeth_fqe *fqe,
@@ -1282,17 +1281,9 @@ static inline void libeth_xdp_prepare_buff(struct libeth_xdp_buff *xdp,
{
const struct page *page = __netmem_to_page(fqe->netmem);
-#ifdef __LIBETH_WORD_ACCESS
- static_assert(offsetofend(typeof(xdp->base), flags) -
- offsetof(typeof(xdp->base), frame_sz) ==
- sizeof(u64));
-
- *(u64 *)&xdp->base.frame_sz = fqe->truesize;
-#else
- xdp_init_buff(&xdp->base, fqe->truesize, xdp->base.rxq);
-#endif
xdp_prepare_buff(&xdp->base, page_address(page) + fqe->offset,
pp_page_to_nmdesc(page)->pp->p.offset, len, true);
+ xdp_init_buff(&xdp->base, fqe->truesize, xdp->base.rxq);
}
/**
diff --git a/include/net/xdp.h b/include/net/xdp.h
index b40f1f96cb11..af60e11b336c 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -85,8 +85,20 @@ 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 flags; /* supported values defined in xdp_buff_flags */
+
+ union {
+ struct {
+ /* frame size to deduce data_hard_end/tailroom */
+ u32 frame_sz;
+ /* supported values defined in xdp_buff_flags */
+ u32 flags;
+ };
+
+#ifdef __LITTLE_ENDIAN
+ /* Used to micro-optimize xdp_init_buff(), don't use directly */
+ u64 frame_sz_flags_init;
+#endif
+ };
};
static __always_inline bool xdp_buff_has_frags(const struct xdp_buff *xdp)
@@ -118,9 +130,19 @@ static __always_inline void xdp_buff_set_frag_pfmemalloc(struct xdp_buff *xdp)
static __always_inline void
xdp_init_buff(struct xdp_buff *xdp, u32 frame_sz, struct xdp_rxq_info *rxq)
{
- xdp->frame_sz = frame_sz;
xdp->rxq = rxq;
+
+#ifdef __LITTLE_ENDIAN
+ /*
+ * Force the compilers to initialize ::flags and assign ::frame_sz with
+ * one write on 64-bit LE architectures as they're often unable to do
+ * it themselves.
+ */
+ xdp->frame_sz_flags_init = frame_sz;
+#else
+ xdp->frame_sz = frame_sz;
xdp->flags = 0;
+#endif
}
static __always_inline void
--
2.51.0
