Re: [dpdk-dev] rte_mbuf structure size in Windows

[Omar Cardona <ocardona@microsoft.com>]

IIRC, it's this issue.  https://bugs.llvm.org/show_bug.cgi?id=24383

-----Original Message-----
From: Tal Shnaiderman <talshn@mellanox.com> 
Sent: Wednesday, May 13, 2020 12:55 AM
To: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>; Thomas Monjalon <thomas@monjalon.net>; pallavi.kadam@intel.com; navasile@linux.microsoft.com; ranjit.menon@intel.com; Harini Ramakrishnan <Harini.Ramakrishnan@microsoft.com>; Omar Cardona <ocardona@microsoft.com>; Dmitry Malloy (MESHCHANINOV) <dmitrym@microsoft.com>; Yohad Tor <yohadt@mellanox.com>
Cc: dev@dpdk.org
Subject: [EXTERNAL] rte_mbuf structure size in Windows

Hi,

I've noticed that there is a difference between the size of rte_mbuf in a Unix build comparing to Windows.

The requirements is for rte_mbuf is to be RTE_CACHE_LINE_MIN_SIZE * 2 bytes however when I'm building it in Windows the size is RTE_CACHE_LINE_MIN_SIZE * 3.

Looks like the diff results from the usage of bit fields inside rte_mbuf, from my testing it looks to me like the usage of 2 different bit fielded types inside rte_mbuf causes additional padding in Windows.

For example from rte_mbuf, the following unions have the same size in Windows and Linux:

	union {
		uint32_t packet_type; 
		// bit fields of type uint32_t will follow
		...
	};...

4 bytes both in Unix and Windows.

	union {
		uint64_t tx_offload;
		// bit fields of type uint64_t will follow
		...
	};

8 bytes both in Unix and Windows.

However when creating a struct containing both unions I'm getting sizeof 16 bytes in Unix and 24 bytes in Windows.

Did someone faced this issue before? Is this a result of different alignment between gcc and clang when bit fields are used? 

Thanks,

Tal