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xdp, libeth: make the xdp_init_buff() micro-optimization generic

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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@kernel.org>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Tested-by: Ramu R <ramu.r@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
This commit is contained in:
Alexander Lobakin
2025-08-26 17:54:55 +02:00
committed by Tony Nguyen
parent c6142e1913
commit 17d370a70b
2 changed files with 26 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files

11
include/net/libeth/xdp.h
View File

@@ -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);
}
/**

28
include/net/xdp.h
View File

@@ -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