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

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

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:mb@smartsharesystems.com]
> Sent: Friday, 10 November 2023 11.44
> 
> > 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@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.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/
> > > > >