Re: [PATCH v3] dmadev: fix structure alignment

[fengchengwen <fengchengwen@huawei.com>]

Reviewed-by: Chengwen Feng <fengchengwen@huawei.com>

On 2024/3/20 15:23, Wenwu Ma wrote:
> 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>
> ---
> v2:
>  - Because of performance drop, adjust the code to
>    no longer demand cache line alignment
> v3:
>  - back to v1 patch
> 
> ---
>  lib/dmadev/rte_dmadev.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/lib/dmadev/rte_dmadev.c b/lib/dmadev/rte_dmadev.c
> index 5953a77bd6..61e106d574 100644
> --- a/lib/dmadev/rte_dmadev.c
> +++ b/lib/dmadev/rte_dmadev.c
> @@ -160,15 +160,25 @@ static int
>  dma_dev_data_prepare(void)
>  {
>  	size_t size;
> +	void *ptr;
>  
>  	if (rte_dma_devices != NULL)
>  		return 0;
>  
> -	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);
>  
>  	return 0;
>  }
>