git: c9e4643da132 - main - iflib: remove transmit prefetching

The branch main has been updated by gallatin:

URL: https://cgit.FreeBSD.org/src/commit/?id=c9e4643da132c50f082a474c540108d7afee2d18

commit c9e4643da132c50f082a474c540108d7afee2d18
Author:     Andrew Gallatin <gallatin@FreeBSD.org>
AuthorDate: 2025-11-11 16:46:28 +0000
Commit:     Andrew Gallatin <gallatin@FreeBSD.org>
CommitDate: 2025-11-11 16:56:54 +0000

    iflib: remove transmit prefetching
    
    Remove prefetching from the transmit path of iflib in the interest of
    increased performance and reduced complexity. Details regarding the
    performance penalties of prefetching can be found in the differential
    review.
    
    Note this prefetching was only done on link speeds of 10Gb/s and
    above, so the change is a no-op (or perhaps slight performance
    improvement simply due to the code simplification) for slower
    interfaces.
    
    Sponsored by: Netflix
    Differential Revision: https://reviews.freebsd.org/D53674
    Reviewed by: kbowling, markj, mjg
---
 sys/net/iflib.c | 46 ++--------------------------------------------
 1 file changed, 2 insertions(+), 44 deletions(-)

diff --git a/sys/net/iflib.c b/sys/net/iflib.c
index d2625da19cd2..ad2be119da7c 100644
--- a/sys/net/iflib.c
+++ b/sys/net/iflib.c
@@ -3449,25 +3449,6 @@ iflib_remove_mbuf(iflib_txq_t txq)
 	return (m);
 }
 
-static inline caddr_t
-calc_next_txd(iflib_txq_t txq, int cidx, uint8_t qid)
-{
-	qidx_t size;
-	int ntxd;
-	caddr_t start, end, cur, next;
-
-	ntxd = txq->ift_size;
-	size = txq->ift_txd_size[qid];
-	start = txq->ift_ifdi[qid].idi_vaddr;
-
-	if (__predict_false(size == 0))
-		return (start);
-	cur = start + size * cidx;
-	end = start + size * ntxd;
-	next = CACHE_PTR_NEXT(cur);
-	return (next < end ? next : start);
-}
-
 /*
  * Pad an mbuf to ensure a minimum ethernet frame size.
  * min_frame_size is the frame size (less CRC) to pad the mbuf to
@@ -3521,37 +3502,22 @@ iflib_encap(iflib_txq_t txq, struct mbuf **m_headp)
 	bus_dma_tag_t		buf_tag;
 	bus_dma_segment_t	*segs;
 	struct mbuf		*m_head, **ifsd_m;
-	void			*next_txd;
 	bus_dmamap_t		map;
 	struct if_pkt_info	pi;
 	int remap = 0;
-	int err, nsegs, ndesc, max_segs, pidx, cidx, next, ntxd;
+	int err, nsegs, ndesc, max_segs, pidx;
 
 	ctx = txq->ift_ctx;
 	sctx = ctx->ifc_sctx;
 	scctx = &ctx->ifc_softc_ctx;
 	segs = txq->ift_segs;
-	ntxd = txq->ift_size;
 	m_head = *m_headp;
 	map = NULL;
 
 	/*
 	 * If we're doing TSO the next descriptor to clean may be quite far ahead
 	 */
-	cidx = txq->ift_cidx;
 	pidx = txq->ift_pidx;
-	if (ctx->ifc_flags & IFC_PREFETCH) {
-		next = (cidx + CACHE_PTR_INCREMENT) & (ntxd - 1);
-		if (!(ctx->ifc_flags & IFLIB_HAS_TXCQ)) {
-			next_txd = calc_next_txd(txq, cidx, 0);
-			prefetch(next_txd);
-		}
-
-		/* prefetch the next cache line of mbuf pointers and flags */
-		prefetch(&txq->ift_sds.ifsd_m[next]);
-		prefetch(&txq->ift_sds.ifsd_map[next]);
-		next = (cidx + CACHE_LINE_SIZE) & (ntxd - 1);
-	}
 	map = txq->ift_sds.ifsd_map[pidx];
 	ifsd_m = txq->ift_sds.ifsd_m;
 
@@ -3737,24 +3703,16 @@ defrag_failed:
 static void
 iflib_tx_desc_free(iflib_txq_t txq, int n)
 {
-	uint32_t qsize, cidx, mask, gen;
+	uint32_t qsize, cidx, gen;
 	struct mbuf *m, **ifsd_m;
-	bool do_prefetch;
 
 	cidx = txq->ift_cidx;
 	gen = txq->ift_gen;
 	qsize = txq->ift_size;
-	mask = qsize - 1;
 	ifsd_m = txq->ift_sds.ifsd_m;
-	do_prefetch = (txq->ift_ctx->ifc_flags & IFC_PREFETCH);
 
 	while (n-- > 0) {
-		if (do_prefetch) {
-			prefetch(ifsd_m[(cidx + 3) & mask]);
-			prefetch(ifsd_m[(cidx + 4) & mask]);
-		}
 		if ((m = ifsd_m[cidx]) != NULL) {
-			prefetch(&ifsd_m[(cidx + CACHE_PTR_INCREMENT) & mask]);
 			if (m->m_pkthdr.csum_flags & CSUM_TSO) {
 				bus_dmamap_sync(txq->ift_tso_buf_tag,
 				    txq->ift_sds.ifsd_tso_map[cidx],