Re: [PATCH v6 5/7] eal: provide option to use compiler memcpy instead of RTE

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2024-10-04 09:52, David Marchand wrote:
> On Fri, Sep 20, 2024 at 12:36 PM Mattias Rönnblom
> <mattias.ronnblom@ericsson.com> wrote:
>>
>> Provide build option to have functions in <rte_memcpy.h> delegate to
>> the standard compiler/libc memcpy(), instead of using the various
>> custom DPDK, handcrafted, per-architecture rte_memcpy()
>> implementations.
>>
>> A new meson build option 'use_cc_memcpy' is added. By default, the
>> traditional, custom DPDK rte_memcpy() implementation is used.
>>
>> The performance benefits of the custom DPDK rte_memcpy()
>> implementations have been diminishing with every compiler release, and
>> with current toolchains the use of a custom memcpy() implementation
>> may even be a liability.
>>
>> An additional benefit of this change is that compilers and static
>> analysis tools have an easier time detecting incorrect usage of
>> rte_memcpy() (e.g., buffer overruns, or overlapping source and
>> destination buffers).
>>
>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>> Acked-by: Morten Brørup <mb@smartsharesystems.com>
> 
> I like this patch and the direction we are taking: stop reinvent
> memcpy and rely on compiler to optimize it.
> 
> I have some comments on the implementation.
> 
> - When I splitted headers in the early days of dpdk, the intention
> with arch-specific headers in EAL was to have them include the generic
> one, in all cases.
> It seems that, over time, x86 rte_memcpy.h (at least) deviated from
> this and stopped including generic/rte_memcpy.h...
> 
> So in this current patch, I expect every arch specific headers first
> include generic/rte_memcpy.h, regardless of any arch-specific define
> coming from the configuration.
> 
> An additional note on this, ARM32 and ARM64 have their own
> implementation in rte_memcpy_32.h resp. rte_memcpy_64.h, and I would
> check RTE_USE_CC_MEMCPY in each of them rather than in the top as
> ARM32 and ARM64 are like two different arches.
> 
> 
> - Now, looking at what was available for arches so far in DPDK:
> * ARM was relying by default on compiler implementation, with specific
> implementations for ARM32 and ARM64 available (see for more details
> below) => possible values (default first) RTE_USE_CC_MEMCPY = true /
> false
> * loongarch was relying on compiler implementation, with no specific
> implementations, => RTE_USE_CC_MEMCPY = true
> * ppc was relying on arch specific implementation, => RTE_USE_CC_MEMCPY = false
> * risc was relying on compiler implementation, with no specific
> implementations, => RTE_USE_CC_MEMCPY = true
> * x86 was relying on arch specific implementation, => RTE_USE_CC_MEMCPY = false
> 
> We can't get a unified default value for a meson option and keep
> compat for all arches (except maybe introduce a "auto" value).
> 

What if you just renamed RTE_USE_CC_MEMCPY to
RTE_ALWAYS_USE_CC_MEMCPY
RTE_FORCE_CC_MEMCPY

Then the naming would better match a scenario where cc memcpy may be the 
only option.

> Plus, disabling RTE_USE_CC_MEMCPY on loongarch and risc makes no
> sense, as there was never a specific implementation.
> 
> My suggestion is to drop the meson option and instead just set
> RTE_USE_CC_MEMCPY in config/$arch/meson.build.
> Testers / interested users may edit config/$arch/meson.build on their own.
> 
> 
> - Additionnally, ARM people have introduced arch-specific
> implementation config options for memcpy in ARM32 resp. ARM64:
> RTE_ARCH_ARM_NEON_MEMCPY resp. RTE_ARCH_ARM64_MEMCPY.
> RTE_USE_CC_MEMCPY can replace those two options (we may keep some
> compat in case someone relied on those defines for arm).
> That removes the need for a RTE_CC_MEMCPY define.
> 
> More comments below:
> 
> [snip]
> 
>> diff --git a/doc/guides/rel_notes/release_24_11.rst b/doc/guides/rel_notes/release_24_11.rst
>> index 0ff70d9057..8be000294d 100644
>> --- a/doc/guides/rel_notes/release_24_11.rst
>> +++ b/doc/guides/rel_notes/release_24_11.rst
>> @@ -55,6 +55,26 @@ New Features
>>        Also, make sure to start the actual text at the margin.
>>        =======================================================
>>
>> +* **Compiler memcpy replaces custom DPDK implementation.**
>> +
>> +  The memory copy functions of ``<rte_memcpy.h>`` now optionally
>> +  delegates to the standard memcpy() function, implemented by the
>> +  compiler and the C runtime (e.g., libc).
>> +
>> +  In this release of DPDK, the handcrafted, per-architecture memory
>> +  copy implementations are still the default. Compiler memcpy is
>> +  enabled by setting the new ``use_cc_memcpy`` build option to true.
>> +
>> +  The performance benefits of the custom DPDK rte_memcpy()
>> +  implementations have been diminishing with every new compiler
>> +  release, and with current toolchains the use of a custom memcpy()
>> +  implementation may even result in worse performance than the
>> +  standard memcpy().
>> +
>> +  An additional benefit of using compiler memcpy is that compilers and
>> +  static analysis tools have an easier time detecting incorrect usage
>> +  of rte_memcpy() (e.g., buffer overruns, or overlapping source and
>> +  destination buffers).
> 
> As explained in the RN comments, an entry should use the form:
> 
>     * **Add a title in the past tense with a full stop.**
> 
>       Add a short 1-2 sentence description in the past tense.
>       The description should be enough to allow someone scanning
>       the release notes to understand the new feature.
> 
> It seems this note is a copy/paste of the commit log, please adjust
> the title and make the description shorter.
> 
>>
>>   Removed Items
>>   -------------
> 
> [snip]
> 
>> diff --git a/lib/eal/include/generic/rte_memcpy.h b/lib/eal/include/generic/rte_memcpy.h
>> index e7f0f8eaa9..cfb0175bd2 100644
>> --- a/lib/eal/include/generic/rte_memcpy.h
>> +++ b/lib/eal/include/generic/rte_memcpy.h
>> @@ -5,12 +5,19 @@
>>   #ifndef _RTE_MEMCPY_H_
>>   #define _RTE_MEMCPY_H_
>>
>> +#ifdef __cplusplus
>> +extern "C" {
>> +#endif
>> +
>>   /**
>>    * @file
>>    *
>>    * Functions for vectorised implementation of memcpy().
>>    */
>>
>> +#include <stdint.h>
>> +#include <string.h>
> 
> I don't think those includes should go in a extern "C" { block.
> 
>> +
>>   /**
>>    * Copy 16 bytes from one location to another using optimised
>>    * instructions. The locations should not overlap.
>> @@ -35,8 +42,6 @@ rte_mov16(uint8_t *dst, const uint8_t *src);
>>   static inline void
>>   rte_mov32(uint8_t *dst, const uint8_t *src);
>>
>> -#ifdef __DOXYGEN__
>> -
> 
> This strange check was added as not all architectures provide
> rte_mov48 (/me slaps Adrien and Thomas).
> I think the CI reported no issue because of a problem in the next
> patch where all that is tested is RTE_USE_CC_MEMCPY = true
> combination.
> 
> Still, the overall goal of this work is to drop the whole rte_memcpy
> thing in the future, so I think we can live with this #ifdef
> __DOXYGEN__ non sense hiding the absence of rte_mov48 in x86...
> 
> 
>>   /**
>>    * Copy 48 bytes from one location to another using optimised
>>    * instructions. The locations should not overlap.
>> @@ -49,8 +54,6 @@ rte_mov32(uint8_t *dst, const uint8_t *src);
>>   static inline void
>>   rte_mov48(uint8_t *dst, const uint8_t *src);
>>
>> -#endif /* __DOXYGEN__ */
>> -
>>   /**
>>    * Copy 64 bytes from one location to another using optimised
>>    * instructions. The locations should not overlap.
>> @@ -87,8 +90,6 @@ rte_mov128(uint8_t *dst, const uint8_t *src);
>>   static inline void
>>   rte_mov256(uint8_t *dst, const uint8_t *src);
>>
>> -#ifdef __DOXYGEN__
>> -
>>   /**
>>    * Copy bytes from one location to another. The locations must not overlap.
>>    *
>> @@ -111,6 +112,52 @@ rte_mov256(uint8_t *dst, const uint8_t *src);
>>   static void *
>>   rte_memcpy(void *dst, const void *src, size_t n);
>>
>> -#endif /* __DOXYGEN__ */
> 
> Removing this DOXYGEN here should be ok.
> CI will tell us.
> 
> 
>> diff --git a/lib/eal/x86/include/meson.build b/lib/eal/x86/include/meson.build
>> index 52d2f8e969..09c2fe2485 100644
>> --- a/lib/eal/x86/include/meson.build
>> +++ b/lib/eal/x86/include/meson.build
>> @@ -16,6 +16,7 @@ arch_headers = files(
>>           'rte_spinlock.h',
>>           'rte_vect.h',
>>   )
>> +
> 
> Unrelated change.
> 
> 
>>   arch_indirect_headers = files(
>>           'rte_atomic_32.h',
>>           'rte_atomic_64.h',
> 
>