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To: Gavin Hu <Gavin.Hu@arm.com>, "dev@dpdk.org" <dev@dpdk.org>
Cc: nd <nd@arm.com>, "david.marchand@redhat.com" <david.marchand@redhat.com>,
"thomas@monjalon.net" <thomas@monjalon.net>,
"ktraynor@redhat.com" <ktraynor@redhat.com>,
"jerinj@marvell.com" <jerinj@marvell.com>,
Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
Ruifeng Wang <Ruifeng.Wang@arm.com>, Phil Yang <Phil.Yang@arm.com>,
Joyce Kong <Joyce.Kong@arm.com>, "stable@dpdk.org" <stable@dpdk.org>,
Olivier MATZ <olivier.matz@6wind.com>,
Konstantin Ananyev <konstantin.ananyev@intel.com>,
Andrew Rybchenko <arybchenko@solarflare.com>
References: <20200303162728.93744-1-gavin.hu@arm.com>
<20200307155629.45021-1-gavin.hu@arm.com>
<4135ab73-75d3-421a-264d-2951fc096133@intel.com>
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From: Ferruh Yigit <ferruh.yigit@intel.com>
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Message-ID: <dffb4554-4719-8838-3001-eebdf7228c48@intel.com>
Date: Mon, 9 Mar 2020 11:29:56 +0000
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Subject: Re: [dpdk-dev] [PATCH v2] mbuf: replace zero-length marker with
unnamed union
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On 3/9/2020 9:45 AM, Gavin Hu wrote:
> Hi Ferruh,
>
>> -----Original Message-----
>> From: Ferruh Yigit <ferruh.yigit@intel.com>
>> Sent: Monday, March 9, 2020 4:55 PM
>> To: Gavin Hu <Gavin.Hu@arm.com>; dev@dpdk.org
>> Cc: nd <nd@arm.com>; david.marchand@redhat.com; thomas@monjalon.net;
>> ktraynor@redhat.com; jerinj@marvell.com; Honnappa Nagarahalli
>> <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
>> <Ruifeng.Wang@arm.com>; Phil Yang <Phil.Yang@arm.com>; Joyce Kong
>> <Joyce.Kong@arm.com>; stable@dpdk.org; Olivier MATZ
>> <olivier.matz@6wind.com>; Konstantin Ananyev
>> <konstantin.ananyev@intel.com>; Andrew Rybchenko
>> <arybchenko@solarflare.com>
>> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: replace zero-length marker with
>> unnamed union
>>
>> On 3/7/2020 3:56 PM, Gavin Hu wrote:
>>> Declaring zero-length arrays in other contexts, including as interior
>>> members of structure objects or as non-member objects, is discouraged.
>>> Accessing elements of zero-length arrays declared in such contexts is
>>> undefined and may be diagnosed.[1]
>>>
>>> Fix by using unnamed union and struct.
>>>
>>> https://bugs.dpdk.org/show_bug.cgi?id=396
>>>
>>> Bugzilla ID: 396
>>>
>>> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>>>
>>> Fixes: 3e6181b07038 ("mbuf: use structure marker from EAL")
>>> Cc: stable@dpdk.org
>>>
>>> Signed-off-by: Gavin Hu <gavin.hu@arm.com>
>>> ---
>>> v2:
>>> * change 'uint64_t rearm_data' to 'uint_64_t rearm_data[1]' to fix
>>> the SFC PMD compiling error on x86. <Kevin Traynor>
>>> ---
>>> lib/librte_mbuf/rte_mbuf_core.h | 54 +++++++++++++++++++--------------
>>> 1 file changed, 32 insertions(+), 22 deletions(-)
>>>
>>> diff --git a/lib/librte_mbuf/rte_mbuf_core.h
>> b/lib/librte_mbuf/rte_mbuf_core.h
>>> index b9a59c879..34cb152e2 100644
>>> --- a/lib/librte_mbuf/rte_mbuf_core.h
>>> +++ b/lib/librte_mbuf/rte_mbuf_core.h
>>> @@ -480,31 +480,41 @@ struct rte_mbuf {
>>> rte_iova_t buf_physaddr; /**< deprecated */
>>> } __rte_aligned(sizeof(rte_iova_t));
>>>
>>> - /* next 8 bytes are initialised on RX descriptor rearm */
>>> - RTE_MARKER64 rearm_data;
>>> - uint16_t data_off;
>>> -
>>> - /**
>>> - * Reference counter. Its size should at least equal to the size
>>> - * of port field (16 bits), to support zero-copy broadcast.
>>> - * It should only be accessed using the following functions:
>>> - * rte_mbuf_refcnt_update(), rte_mbuf_refcnt_read(), and
>>> - * rte_mbuf_refcnt_set(). The functionality of these functions (atomic,
>>> - * or non-atomic) is controlled by the
>> CONFIG_RTE_MBUF_REFCNT_ATOMIC
>>> - * config option.
>>> - */
>>> RTE_STD_C11
>>> union {
>>> - rte_atomic16_t refcnt_atomic; /**< Atomically accessed
>> refcnt */
>>> - /** Non-atomically accessed refcnt */
>>> - uint16_t refcnt;
>>> - };
>>> - uint16_t nb_segs; /**< Number of segments. */
>>> + /* next 8 bytes are initialised on RX descriptor rearm */
>>> + uint64_t rearm_data[1];
>> We are using zero length array as markers only and know what we are doing
>> with them,
>> what would you think disabling the warning instead of increasing the
>> complexity
>> in mbuf struct?
> Okay, I will add -Wno-zero-length-bounds to the compiler toolchain flags.
This would be my preference but I would like to get more input, can you please
for more comments before changing the implementation in case there are some
strong opinion on it?