Dear Ruifeng, +CC: all CPU architecture maintainers, I'm trying to figure out the requirements for supporting unaligned memory access in DPDK (specifically the ring library), and need your expert feedback.
The #define RTE_ARCH_STRICT_ALIGN - which is undocumented, but probably means that CPU memory access must be aligned - is only set by "generic_aarch32". So we will assume that all other CPU architectures supported by DPDK can access unaligned memory. As discussed in this thread, "generic_aarch32" has special requirements for performing 64-bit load/store at unaligned addresses. Now comes the big question: Can "generic_aarch32" perform 32-bit load/store at unaligned addresses without similar special requirements? Then the ring library already supports unaligned 32-bit objects, and doesn't need to be fixed in this regard. Med venlig hilsen / Kind regards, -Morten Brørup > From: Morten Brørup [mailto:m...@smartsharesystems.com] > Sent: Friday, 10 November 2023 11.44 > > > From: Konstantin Ananyev [mailto:konstantin.anan...@huawei.com] > > Sent: Friday, 10 November 2023 10.45 > > > > > From: Morten Brørup <m...@smartsharesystems.com> > > > Sent: Friday, November 10, 2023 9:34 AM > > > > > > +CC Gavin, reviewed the test case > > > > > > > From: Ruifeng Wang [mailto:ruifeng.w...@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-boun...@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@smarts > erver.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.y...@arm.com> > > > > >> Reviewed-by: Ruifeng Wang <ruifeng.w...@arm.com> > > > > >> Reviewed-by: Honnappa Nagarahalli > <honnappa.nagaraha...@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.y...@arm.com/ > > > > >
