svn commit: r366014 - in stable/12/sys: dev/bnxt dev/vmware/vmxnet3 net
Vincenzo Maffione
vmaffione at FreeBSD.org
Tue Sep 22 21:43:45 UTC 2020
Author: vmaffione
Date: Tue Sep 22 21:43:43 2020
New Revision: 366014
URL: https://svnweb.freebsd.org/changeset/base/366014
Log:
MFC r365061
iflib: leave only 1 receive descriptor unused
The pidx argument of isc_rxd_flush() indicates which is the last valid
receive descriptor to be used by the NIC. However, current code has
multiple issues:
- Intel drivers write pidx to their RDT register, which means that
NICs will only use the descriptors up to pidx-1 (modulo ring size N),
and won't actually use the one pointed by pidx. This does not break
reception, but it is anyway confusing and suboptimal (the NIC will
actually see only N-2 descriptors as available, rather than N-1).
Other drivers (if_vmx, if_bnxt, if_mgb) adhere to this semantic.
- The semantic used by Intel (RDT is one descriptor past the last
valid one) is used by most (if not all) NICs, and it is also used
on the TX side (also in iflib). Since iflib is not currently
using this semantic for RX, it must decrement fl->ifl_pidx
(modulo N) before calling isc_rxd_flush(), and then the
per-driver callback implementation must increment the index
again (to match the real semantic). This is confusing and suboptimal.
- The iflib refill function is also called at initialization.
However, in case the ring size is smaller than 128 (e.g. if_mgb),
the refill function will actually prepare all the receive
descriptors (N), without leaving one unused, as most of NICs assume
(e.g. to avoid RDT to overrun RDH). I can speculate that the code
looks like this right now because this issue showed up during
testing (e.g. with if_mgb), and it was easy to workaround by
decrementing pidx before isc_rxd_flush().
The goal of this change is to simplify the code (removing a bunch
of instructions from the RX fast path), and to make the semantic of
isc_rxd_flush() consistent across drivers. To achieve this, we:
- change the semantics of the pidx argument to the usual one (that
is the index one past the last valid one), so that both iflib and
drivers avoid the decrement/increment dance.
- fix the initialization code to prepare at most N-1 descriptors.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D26191
Modified:
stable/12/sys/dev/bnxt/bnxt_txrx.c
stable/12/sys/dev/vmware/vmxnet3/if_vmx.c
stable/12/sys/net/iflib.c
Directory Properties:
stable/12/ (props changed)
Modified: stable/12/sys/dev/bnxt/bnxt_txrx.c
==============================================================================
--- stable/12/sys/dev/bnxt/bnxt_txrx.c Tue Sep 22 21:16:03 2020 (r366013)
+++ stable/12/sys/dev/bnxt/bnxt_txrx.c Tue Sep 22 21:43:43 2020 (r366014)
@@ -316,10 +316,9 @@ bnxt_isc_rxd_flush(void *sc, uint16_t rxqid, uint8_t f
if (softc->rx_cp_rings[rxqid].cons != UINT32_MAX)
BNXT_CP_IDX_DISABLE_DB(&softc->rx_cp_rings[rxqid].ring,
softc->rx_cp_rings[rxqid].cons);
- /* We're given the last filled RX buffer here, not the next empty one */
- BNXT_RX_DB(rx_ring, RING_NEXT(rx_ring, pidx));
+ BNXT_RX_DB(rx_ring, pidx);
/* TODO: Cumulus+ doesn't need the double doorbell */
- BNXT_RX_DB(rx_ring, RING_NEXT(rx_ring, pidx));
+ BNXT_RX_DB(rx_ring, pidx);
return;
}
Modified: stable/12/sys/dev/vmware/vmxnet3/if_vmx.c
==============================================================================
--- stable/12/sys/dev/vmware/vmxnet3/if_vmx.c Tue Sep 22 21:16:03 2020 (r366013)
+++ stable/12/sys/dev/vmware/vmxnet3/if_vmx.c Tue Sep 22 21:43:43 2020 (r366014)
@@ -1744,13 +1744,6 @@ vmxnet3_isc_rxd_flush(void *vsc, uint16_t rxqid, uint8
else
r = VMXNET3_BAR0_RXH2(rxqid);
- /*
- * pidx is the index of the last descriptor with a buffer the device
- * can use, and the device needs to be told which index is one past
- * that.
- */
- if (++pidx == rxr->vxrxr_ndesc)
- pidx = 0;
vmxnet3_write_bar0(sc, r, pidx);
}
Modified: stable/12/sys/net/iflib.c
==============================================================================
--- stable/12/sys/net/iflib.c Tue Sep 22 21:16:03 2020 (r366013)
+++ stable/12/sys/net/iflib.c Tue Sep 22 21:43:43 2020 (r366014)
@@ -2030,7 +2030,8 @@ _rxq_refill_cb(void *arg, bus_dma_segment_t *segs, int
* @count: the number of new buffers to allocate
*
* (Re)populate an rxq free-buffer list with up to @count new packet buffers.
- * The caller must assure that @count does not exceed the queue's capacity.
+ * The caller must assure that @count does not exceed the queue's capacity
+ * minus one (since we always leave a descriptor unavailable).
*/
static uint8_t
iflib_fl_refill(if_ctx_t ctx, iflib_fl_t fl, int count)
@@ -2045,6 +2046,8 @@ iflib_fl_refill(if_ctx_t ctx, iflib_fl_t fl, int count
int err, frag_idx, i, idx, n, pidx;
qidx_t credits;
+ MPASS(count <= fl->ifl_size - fl->ifl_credits - 1);
+
sd_m = fl->ifl_sds.ifsd_m;
sd_map = fl->ifl_sds.ifsd_map;
sd_cl = fl->ifl_sds.ifsd_cl;
@@ -2151,15 +2154,10 @@ iflib_fl_refill(if_ctx_t ctx, iflib_fl_t fl, int count
fl->ifl_credits = credits;
}
DBG_COUNTER_INC(rxd_flush);
- if (fl->ifl_pidx == 0)
- pidx = fl->ifl_size - 1;
- else
- pidx = fl->ifl_pidx - 1;
-
bus_dmamap_sync(fl->ifl_ifdi->idi_tag, fl->ifl_ifdi->idi_map,
BUS_DMASYNC_PREREAD | BUS_DMASYNC_PREWRITE);
ctx->isc_rxd_flush(ctx->ifc_softc, fl->ifl_rxq->ifr_id,
- fl->ifl_id, pidx);
+ fl->ifl_id, fl->ifl_pidx);
if (__predict_true(bit_test(fl->ifl_rx_bitmap, frag_idx))) {
fl->ifl_fragidx = frag_idx + 1;
if (fl->ifl_fragidx == fl->ifl_size)
@@ -2175,7 +2173,17 @@ iflib_fl_refill(if_ctx_t ctx, iflib_fl_t fl, int count
static inline uint8_t
iflib_fl_refill_all(if_ctx_t ctx, iflib_fl_t fl)
{
- /* we avoid allowing pidx to catch up with cidx as it confuses ixl */
+ /*
+ * We leave an unused descriptor to avoid pidx to catch up with cidx.
+ * This is important as it confuses most NICs. For instance,
+ * Intel NICs have (per receive ring) RDH and RDT registers, where
+ * RDH points to the next receive descriptor to be used by the NIC,
+ * and RDT for the next receive descriptor to be published by the
+ * driver to the NIC (RDT - 1 is thus the last valid one).
+ * The condition RDH == RDT means no descriptors are available to
+ * the NIC, and thus it would be ambiguous if it also meant that
+ * all the descriptors are available to the NIC.
+ */
int32_t reclaimable = fl->ifl_size - fl->ifl_credits - 1;
#ifdef INVARIANTS
int32_t delta = fl->ifl_size - get_inuse(fl->ifl_size, fl->ifl_cidx, fl->ifl_pidx, fl->ifl_gen) - 1;
@@ -2280,12 +2288,15 @@ iflib_fl_setup(iflib_fl_t fl)
fl->ifl_zone = m_getzone(fl->ifl_buf_size);
- /* avoid pre-allocating zillions of clusters to an idle card
- * potentially speeding up attach
+ /*
+ * Avoid pre-allocating zillions of clusters to an idle card
+ * potentially speeding up attach. In any case make sure
+ * to leave a descriptor unavailable. See the comment in
+ * iflib_fl_refill_all().
*/
- (void)iflib_fl_refill(ctx, fl, min(128, fl->ifl_size));
- MPASS(min(128, fl->ifl_size) == fl->ifl_credits);
- if (min(128, fl->ifl_size) != fl->ifl_credits)
+ MPASS(fl->ifl_size > 0);
+ (void)iflib_fl_refill(ctx, fl, min(128, fl->ifl_size - 1));
+ if (min(128, fl->ifl_size - 1) != fl->ifl_credits)
return (ENOBUFS);
/*
* handle failure
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