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 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.


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