RE: [PATCH v2] dmadev: fix structure alignment

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

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)

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