RE: [PATCH v2] dmadev: fix structure alignment

["Ma, WenwuX" <wenwux.ma@intel.com>]

Hi Chengwen

> -----Original Message-----
> From: fengchengwen <fengchengwen@huawei.com>
> Sent: Friday, March 15, 2024 4:32 PM
> To: Ma, WenwuX <wenwux.ma@intel.com>; dev@dpdk.org
> Cc: Jiale, SongX <songx.jiale@intel.com>; stable@dpdk.org
> Subject: Re: [PATCH v2] dmadev: fix structure alignment
> 
> Hi Wenwu,
> 
> On 2024/3/15 15:44, Ma, WenwuX wrote:
> > Hi Chengwen,
> >
> >> -----Original Message-----
> >> From: Ma, WenwuX
> >> Sent: Friday, March 15, 2024 2:26 PM
> >> To: fengchengwen <fengchengwen@huawei.com>; dev@dpdk.org
> >> Cc: Jiale, SongX <songx.jiale@intel.com>; stable@dpdk.org
> >> Subject: RE: [PATCH v2] dmadev: fix structure alignment
> >>
> >> Hi Chengwen,
> >>
> >>> -----Original Message-----
> >>> From: fengchengwen <fengchengwen@huawei.com>
> >>> Sent: Friday, March 15, 2024 2:06 PM
> >>> To: Ma, WenwuX <wenwux.ma@intel.com>; dev@dpdk.org
> >>> Cc: Jiale, SongX <songx.jiale@intel.com>; stable@dpdk.org
> >>> Subject: Re: [PATCH v2] dmadev: fix structure alignment
> >>>
> >>> Hi Wenwu,
> >>>
> >>> On 2024/3/15 9:43, Wenwu Ma wrote:
> >>>> The structure rte_dma_dev needs only 8 byte alignment.
> >>>> This patch replaces __rte_cache_aligned of rte_dma_dev with
> >>>> __rte_aligned(8).
> >>>>
> >>>> Fixes: b36970f2e13e ("dmadev: introduce DMA device library")
> >>>> Cc: stable@dpdk.org
> >>>>
> >>>> Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
> >>>> ---
> >>>> v2:
> >>>>  - Because of performance drop, adjust the code to
> >>>>    no longer demand cache line alignment
> >>>
> >>> Which two versions observed performance drop? And which benchmark
> >>> observed drop?
> >>> Could you provide more information?
> >>>
> >>>>
> >> V1 patch:
> >>
> https://patches.dpdk.org/project/dpdk/patch/20240308053711.1260154-
> >> 1-wenwux.ma@intel.com/
> >>
> >> To view detailed results, visit:
> >> https://lab.dpdk.org/results/dashboard/patchsets/29472/
> >>
> >>>> ---
> >>>>  lib/dmadev/rte_dmadev_pmd.h | 2 +-
> >>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/lib/dmadev/rte_dmadev_pmd.h
> >>> b/lib/dmadev/rte_dmadev_pmd.h
> >>>> index 58729088ff..b569bb3502 100644
> >>>> --- a/lib/dmadev/rte_dmadev_pmd.h
> >>>> +++ b/lib/dmadev/rte_dmadev_pmd.h
> >>>> @@ -122,7 +122,7 @@ enum rte_dma_dev_state {
> >>>>   * @internal
> >>>>   * The generic data structure associated with each DMA device.
> >>>>   */
> >>>> -struct __rte_cache_aligned rte_dma_dev {
> >>>> +struct __rte_aligned(8) rte_dma_dev {
> >>>
> >>> The DMA fast-path was implemented by struct rte_dma_fp_objs, which
> >>> is not rte_dma_dev? So why is it a problem here?
> >>>
> >>> Thanks
> >>>
> >> The DMA device object is expected to align cache line, so clang will
> >> use “vmovaps” assembly instruction,
> >>
> >> And the instruction demands 16 bytes alignment or will cause segment
> >> fault in some environments.
> >>
> > Test case:
> > 1. compile dpdk
> > rm -rf x86_64-native-linuxapp-clang
> > CC=clang meson -Denable_kmods=True -Dlibdir=lib
> > --default-library=static x86_64-native-linuxapp-clang ninja -C
> > x86_64-native-linuxapp-clang -j 72 2. start dpdk-test
> > /root/dpdk/x86_64-native-linuxapp-clang/app/dpdk-test -l 0-39
> > --vdev=dma_skeleton -a 31:00.0 -a 31:00.1 -a 31:00.2 -a 31:00.3 （Note:
> > If it cannot be reproduced, please try using a different core）
> > 3. exit dpdk-test
> > RTE>>quit
> > Segmentation fault (core dumped)
> 
> I will try to reproduce, but still a question: does above test has already merged
> your patch [1] or the current main branch code has this problem?
> 
> [1]
> https://patches.dpdk.org/project/dpdk/patch/20240308053711.1260154-
> 1-wenwux.ma@intel.com/
> 
> Thanks
> 
the current main branch code has this problem.

Both patch v1 and v2 are able to solve this problem, but v1 has a performance issue.

> >
> >>
> >>>>  	/** Device info which supplied during device initialization. */
> >>>>  	struct rte_device *device;
> >>>>  	struct rte_dma_dev_data *data; /**< Pointer to shared device data.
> >>>> */
> >>>>