RE: [PATCH v3] dmadev: fix structure alignment

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

Hi, Thomas

> -----Original Message-----
> From: Thomas Monjalon <thomas@monjalon.net>
> Sent: Wednesday, March 20, 2024 7:37 PM
> To: fengchengwen@huawei.com; Ma, WenwuX <wenwux.ma@intel.com>
> Cc: dev@dpdk.org; Jiale, SongX <songx.jiale@intel.com>; stable@dpdk.org
> Subject: Re: [PATCH v3] dmadev: fix structure alignment
> 
> 20/03/2024 08:23, Wenwu Ma:
> > The structure rte_dma_dev needs to be aligned to the cache line, but
> > the return value of malloc may not be aligned to the cache line. When
> > we use memset to clear the rte_dma_dev object, it may cause a
> > segmentation fault in clang-x86-platform.
> >
> > This is because clang uses the "vmovaps" assembly instruction for
> > memset, which requires that the operands (rte_dma_dev objects) must
> > aligned on a 16-byte boundary or a general-protection exception (#GP)
> > is generated.
> >
> > Therefore, either additional memory is applied for re-alignment, or
> > the rte_dma_dev object does not require cache line alignment. The
> > patch chooses the former option to fix the issue.
> >
> > Fixes: b36970f2e13e ("dmadev: introduce DMA device library")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
> [..]
> > -	size = dma_devices_max * sizeof(struct rte_dma_dev);
> > -	rte_dma_devices = malloc(size);
> > -	if (rte_dma_devices == NULL)
> > +	/* The dma device object is expected to align cacheline, but
> > +	 * the return value of malloc may not be aligned to the cache line.
> > +	 * Therefore, extra memory is applied for realignment.
> > +	 * note: We do not call posix_memalign/aligned_alloc because it is
> > +	 * version dependent on libc.
> > +	 */
> > +	size = dma_devices_max * sizeof(struct rte_dma_dev) +
> > +		RTE_CACHE_LINE_SIZE;
> > +	ptr = malloc(size);
> > +	if (ptr == NULL)
> >  		return -ENOMEM;
> > -	memset(rte_dma_devices, 0, size);
> > +	memset(ptr, 0, size);
> > +
> > +	rte_dma_devices = RTE_PTR_ALIGN(ptr, RTE_CACHE_LINE_SIZE);
> 
> Why not using aligned_alloc()?
> https://en.cppreference.com/w/c/memory/aligned_alloc
> 
> 
because it is version dependent on libc.