Re: [dpdk-dev] [PATCH v7 02/17] lib/ring: apis to support configurable element size

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> <snip>
> > > > > > > > +
> > > > > > > > +static __rte_always_inline void enqueue_elems_128(struct
> > > > > > > > +rte_ring *r, uint32_t prod_head, const void *obj_table,
> > > > > > > > +uint32_t n) { unsigned int i; const uint32_t size =
> > > > > > > > +r->size; uint32_t idx = prod_head & r->mask; __uint128_t
> > > > > > > > +*ring = (__uint128_t *)&r[1]; const __uint128_t *obj =
> > > > > > > > +(const __uint128_t *)obj_table; if (likely(idx + n < size))
> > > > > > > > +{ for (i = 0; i < (n & ~0x1); i += 2, idx += 2) { ring[idx]
> > > > > > > > += obj[i]; ring[idx + 1] = obj[i + 1];
> > > > > > >
> > > > > > >
> > > > > > > AFAIK, that implies 16B aligned obj_table...
> > > > > > > Would it always be the case?
> > > > > > I am not sure from the compiler perspective.
> > > > > > At least on Arm architecture, unaligned access (address that is
> > > > > > accessed is not aligned to the size of the data element being
> > > > > > accessed) will result in faults or require additional cycles.
> > > > > > So, aligning on
> > > > 16B should be fine.
> > > > > Further, I would be changing this to use 'rte_int128_t' as
> > > > > '__uint128_t' is
> > > > not defined on 32b systems.
> > > >
> > > > What I am trying to say: with this code we imply new requirement for
> > > > elems
> > > The only existing use case in DPDK for 16B is the event ring. The event ring
> > already does similar kind of copy (using 'struct rte_event').
> > > So, there is no change in expectations for event ring.
> > > For future code, I think this expectation should be fine since it allows for
> > optimal code.
> > >
> > > > in the ring: when sizeof(elem)==16 it's alignment also has to be at least
> > 16.
> > > > Which from my perspective is not ideal.
> > > Any reasoning?
> >
> > New implicit requirement and inconsistency.
> > Code like that:
> >
> > struct ring_elem {uint64_t a, b;};
> > ....
> > struct ring_elem elem;
> > rte_ring_dequeue_elem(ring, &elem, sizeof(elem));
> >
> > might cause a crash.
> The alignment here is 8B. Assuming that instructions generated will require 16B alignment, it will result in a crash, if configured to generate
> exception.
> But, these instructions are not atomic instructions. At least on aarch64, unaligned access will not result in an exception for non-atomic
> loads/stores. I believe it is the same behavior for x86 as well.

On IA, there are 2 types of 16B load/store instructions: aligned and unaligned.
Aligned are a bit faster, but will cause an exception if used on non 16B aligned address. 
As you using uint128_t * compiler will assume that both src and dst are 16B aligned
and might generate code with aligned instructions.

> 
> > While exactly the same code with:
> >
> > struct ring_elem {uint64_t a, b, c;}; OR struct ring_elem {uint64_t a, b, c, d;};
> >
> > will work ok.
> The alignment for these structures is still 8B. Are you saying this will work because these will be copied using pointer to uint32_t (whose
> alignment is 4B)?

Yes, as we doing uint32_t copies, compiler can't assume the data will be 16B aligned
and will use unaligned instructions.

> 
> >
> > >
> > > > Note that for elem sizes > 16 (24, 32), there is no such constraint.
> > > The rest of them need to be aligned on 4B boundary. However, this should
> > not affect the existing code.
> > > The code for 8B and 16B is kept as is to ensure the performance is not
> > affected for the existing code.
> <snip>