[PATCH v3] dmadev: fix structure alignment

[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 at dpdk.org

Signed-off-by: Wenwu Ma <wenwux.ma at 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;
 }
-- 
2.25.1