[dpdk-dev] [PATCH v2] mbuf: fix 64bit address alignment in 32-bit builds

[Bruce Richardson <bruce.richardson@intel.com>]

On i686 builds, the uin64_t type is 64-bits in size but is aligned to
32-bits only. This causes mbuf fields for rearm_data to not be 16-byte
aligned on 32-bit builds, which causes errors with some vector PMDs which
expect the rearm data to be aligned as on 64-bit.

Given that we cannot use the extra space in the data structures anyway, as
it's already used on 64-bit builds, we can just force alignment of the
physical address in the mbuf to 8-bytes in all cases. This has no effect on
64-bit systems, but fixes the updated PMDs on 32-bit.

Fixes: f4356d7ca168 ("net/i40e: eliminate mbuf write on rearm")
Fixes: f160666a1073 ("net/ixgbe: eliminate mbuf write on rearm")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
v2: change alignment fix from being for all phys_addr_t vars to just
the one in the mbuf structure. This is a lower risk fix. Additional
patches promised to put in build-checks for alignment in vpmds will
be sent separately.
---
 lib/librte_mbuf/rte_mbuf.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 9dd8e80..66f8013 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -403,7 +403,13 @@ struct rte_mbuf {
 	MARKER cacheline0;
 
 	void *buf_addr;           /**< Virtual address of segment buffer. */
-	phys_addr_t buf_physaddr; /**< Physical address of segment buffer. */
+	/**
+	 * Physical address of segment buffer.
+	 * Force alignment to 8-bytes, so as to ensure we have the exact
+	 * same mbuf cacheline0 layout for 32-bit and 64-bit. This makes
+	 * working on vector drivers easier.
+	 */
+	phys_addr_t buf_physaddr __rte_aligned(sizeof(phys_addr_t));
 
 	/* next 8 bytes are initialised on RX descriptor rearm */
 	MARKER64 rearm_data;
-- 
2.9.3