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Subject: RE: [PATCH v4 20/35] net/intel: write descriptors using non-volatile
 pointers
Date: Tue, 10 Feb 2026 10:28:10 +0100
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35F656F9@smartserver.smartshare.dk>
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Thread-Topic: [PATCH v4 20/35] net/intel: write descriptors using non-volatile
 pointers
Thread-Index: AdyabC5k7cVYs9VwQzyejkeBJh1rJwAAomsA
References: <20251219172548.2660777-1-bruce.richardson@intel.com>
 <20260209164538.1428499-1-bruce.richardson@intel.com>
 <20260209164538.1428499-21-bruce.richardson@intel.com>
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From: =?iso-8859-1?Q?Morten_Br=F8rup?= <mb@smartsharesystems.com>
To: "Bruce Richardson" <bruce.richardson@intel.com>
Cc: <dev@dpdk.org>
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> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Tuesday, 10 February 2026 10.04
>=20
> On Tue, Feb 10, 2026 at 12:08:44AM +0100, Morten Br=F8rup wrote:
> > > +static inline void
> > > +write_txd(volatile void *txd, uint64_t qw0, uint64_t qw1)
> > > +{
> > > +	uint64_t *txd_qw =3D __rte_assume_aligned(RTE_CAST_PTR(void *,
> > > txd), 16);
> > > +
> > > +	txd_qw[0] =3D rte_cpu_to_le_64(qw0);
> > > +	txd_qw[1] =3D rte_cpu_to_le_64(qw1);
> > > +}
> >
> > How about using __rte_aligned() instead, something like this
> (untested):
> >
> > struct __rte_aligned(16) txd_t {
> > 	uint64_t	qw0;
> > 	uint64_t	qw1;
> > };
>=20
> I can see if this works for us...
>=20
> >
> > *RTE_CAST_PTR(volatile struct txd_t *, txd) =3D {
> rte_cpu_to_le_64(qw0),
> > rte_cpu_to_le_64(qw1) };
> >
> >
> > And why strip the "volatile"?
> >
>=20
> For the descriptor writes, it doesn't matter the order in which the
> descriptors and the descriptor fields are actually written, since the
> NIC
> relies upon the tail pointer update - which includes a fence - to
> inform it
> of when the descriptors are ready. The volatile is necessary for =
reads,
> though, which is why the ring is marked as such, but for Tx it =
prevents
> the
> compiler from opportunistically e.g. converting two 64-byte writes =
into
> a
> 128-byte write.

Makes sense.
Suggest that you spread out a few comments about this at the relevant =
locations in the source code.