RE: [dpdk-dev] [PATCH] ring: fix unaligned memory access on aarch32

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Konstantin Ananyev [mailto:konstantin.ananyev@huawei.com]
> Sent: Friday, 10 November 2023 10.45
> 
> > From: Morten Brørup <mb@smartsharesystems.com>
> > Sent: Friday, November 10, 2023 9:34 AM
> >
> > +CC Gavin, reviewed the test case
> >
> > > From: Ruifeng Wang [mailto:Ruifeng.Wang@arm.com]
> > > Sent: Friday, 10 November 2023 09.40
> > >
> > > On 2023/11/4 8:04 AM, Morten Brørup wrote:
> > > > I have for a long time now wondered why the ring functions for
> > > enqueue/dequeue of 64-bit objects supports unaligned addresses, and
> now
> > > I finally found the patch introducing it.
> > > >
> > > >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Phil Yang
> > > >> Sent: Monday, 9 March 2020 18.20
> > > >>
> > > >> The 32-bit arm machine doesn't support unaligned memory access.
> It
> > > >> will cause a bus error on aarch32 with the custom element size
> ring.
> > > >>
> > > >> Thread 1 "test" received signal SIGBUS, Bus error.
> > > >> __rte_ring_enqueue_elems_64 (n=1, obj_table=0xf5edfe41,
> prod_head=0,
> > > \
> > > >> r=0xf5edfb80) at /build/dpdk/build/include/rte_ring_elem.h:177
> > > >> 177                             ring[idx++] = obj[i++];
> > > >
> > > > Which test is this? Why is it using an unaligned array of 64-bit
> > > objects? (Notice that obj_table=0xf5edfe41.)
> > >
> > > The test case is:
> > >
> https://elixir.bootlin.com/dpdk/latest/source/app/test/test_ring.c#L112
> > > 1
> > > This case deliberately use unaligned objects.
> >
> > Thank you, Ruifeng.
> >
> > Honnappa, I see you signed off on the patch introducing the test for
> unaligned objects:
> >
> http://git.dpdk.org/dpdk/commit/app/test/test_ring.c?id=a9fe152363e283d
> 4c590ab8e8d51396f2ffab9ff
> >
> > What was the rationale behind testing support for unaligned object
> pointers? Did any applications/customers use unaligned object
> > pointers, or is it a purely academic test case?
> >
> > >
> > > >
> > > > Nobody in their right mind would use an unaligned array of 64-bit
> > > objects. You can only create such an array if you force the
> compiler to
> > > prevent automatic alignment! And all the functions in your
> application
> > > using this array would also need to support unaligned addressing of
> > > these objects.
> 
> It could be just one elem, not an array.
> And the user can use 'packed' struct or so...
> Agree, not a common case, but probably still possible.

Very good point, Konstantin. A single unaligned object in a packed structure is not as exotic as an unaligned array of objects (which I consider completely unrealistic).

If anyone is using an architecture which doesn't support unaligned accesses, I would expect them to completely avoid using unaligned objects. But perhaps you are right; however unlikely, it might happen.

If we think this is a real use case, should we add support for unaligned 32 bit objects?
(128 bit objects already support unaligned access; they are type casted to void pointer and accessed using memcpy().)

And how about the stack library, should it support unaligned objects too?

> 
> > > >
> > > > This seems extremely exotic, and not something any real
> application
> > > would do!
> > > >
> > > > I would like to revert this patch for performance reasons.
> 
> Morten, could you probably explain first the problems you encountered
> with this patch?
> You mention about 'performance reasons', so did you notice any real
> slowdown?

Please check my reply to the same question here:
http://inbox.dpdk.org/dev/98CBD80474FA8B44BF855DF32C47DC35E9EFD3@smartserver.smartshare.dk/

> 
> >
> > I could add an RTE_ASSERT() to verify that the pointer is aligned,
> for debugging purposes.
> >
> > > >
> > > >>
> > > >> Fixes: cc4b218790f6 ("ring: support configurable element size")
> > > >>
> > > >> Signed-off-by: Phil Yang <phil.yang@arm.com>
> > > >> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > >> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > > >> ---
> > > >>   lib/librte_ring/rte_ring_elem.h | 4 ++--
> > > >>   1 file changed, 2 insertions(+), 2 deletions(-)
> > > >>
> > > >> diff --git a/lib/librte_ring/rte_ring_elem.h
> > > >> b/lib/librte_ring/rte_ring_elem.h
> > > >> index 3976757..663addc 100644
> > > >> --- a/lib/librte_ring/rte_ring_elem.h
> > > >> +++ b/lib/librte_ring/rte_ring_elem.h
> > > >> @@ -160,7 +160,7 @@ __rte_ring_enqueue_elems_64(struct rte_ring
> *r,
> > > >> uint32_t prod_head,
> > > >>   	const uint32_t size = r->size;
> > > >>   	uint32_t idx = prod_head & r->mask;
> > > >>   	uint64_t *ring = (uint64_t *)&r[1];
> > > >> -	const uint64_t *obj = (const uint64_t *)obj_table;
> > > >> +	const unaligned_uint64_t *obj = (const unaligned_uint64_t
> > > >> *)obj_table;
> > > >>   	if (likely(idx + n < size)) {
> > > >>   		for (i = 0; i < (n & ~0x3); i += 4, idx += 4) {
> > > >>   			ring[idx] = obj[i];
> > > >> @@ -294,7 +294,7 @@ __rte_ring_dequeue_elems_64(struct rte_ring
> *r,
> > > >> uint32_t prod_head,
> > > >>   	const uint32_t size = r->size;
> > > >>   	uint32_t idx = prod_head & r->mask;
> > > >>   	uint64_t *ring = (uint64_t *)&r[1];
> > > >> -	uint64_t *obj = (uint64_t *)obj_table;
> > > >> +	unaligned_uint64_t *obj = (unaligned_uint64_t *)obj_table;
> > > >>   	if (likely(idx + n < size)) {
> > > >>   		for (i = 0; i < (n & ~0x3); i += 4, idx += 4) {
> > > >>   			obj[i] = ring[idx];
> > > >> --
> > > >> 2.7.4
> > > >>
> > > >
> > > > References:
> > > >
> > >
> https://git.dpdk.org/dpdk/commit/lib/librte_ring/rte_ring_elem.h?id=3ba
> > > 51478a3ab3132c33effc8b132641233275b36
> > > > https://patchwork.dpdk.org/project/dpdk/patch/1583774395-10233-1-
> git-
> > > send-email-phil.yang@arm.com/
> > > >