Re: [PATCH v4 20/35] net/intel: write descriptors using non-volatile pointers

Bruce Richardson
2026-02-10T09:03:52Z

On Tue, Feb 10, 2026 at 12:08:44AM +0100, Morten Brørup wrote:
> > +static inline void
> > +write_txd(volatile void *txd, uint64_t qw0, uint64_t qw1)
> > +{
> > + uint64_t *txd_qw = __rte_assume_aligned(RTE_CAST_PTR(void *,
> > txd), 16);
> > +
> > + txd_qw[0] = rte_cpu_to_le_64(qw0);
> > + txd_qw[1] = 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;
> };

I can see if this works for us...

>
> *RTE_CAST_PTR(volatile struct txd_t *, txd) = { rte_cpu_to_le_64(qw0),
> rte_cpu_to_le_64(qw1) };
>
>
> And why strip the "volatile"?
>

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.

/Bruce