Re: [dpdk-dev] [PATCH v4] ethdev: extend flow metadata

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 10/30/19 6:49 PM, Olivier Matz wrote:
> Hi,
>
> On Wed, Oct 30, 2019 at 10:35:16AM +0300, Andrew Rybchenko wrote:
>> @Olivier, please, take a look at the end of the mail.
>>
> (...)
>
>> On 10/29/19 8:19 PM, Slava Ovsiienko wrote:
>>>>> +};
>>>>> +
>>>>> +/* Mbuf dynamic field offset for metadata. */ extern int
>>>>> +rte_flow_dynf_metadata_offs;
>>>>> +
>>>>> +/* Mbuf dynamic field flag mask for metadata. */ extern uint64_t
>>>>> +rte_flow_dynf_metadata_mask;
>>>> These two global variables look frightening to me.
>>>> It does not look good to me.
>>> For me too. But we need the performance, these ones are
>>> intended for usage in datapath, any overhead is painful.
>> @Olivier, could you share your thoughts, please.
> Having a global variable looks unavoidable to me, if we want
> performance.
>
> An alternative can be to use static global variables in every file that
> use this dynamic field, and call the register method from it. But I
> don't think it would scale if a dynamic field is widely used.

Yes, I see.

> Why does it look frigthening to you?

It is just a good/bad design feeling. No specific technical reasons 
right now.
Let's try and take a look how it goes.

> The constraint is that before using this variable, the register function
> has to be called. I don't think there are race conditions, because the
> field/flag registration is lock protected and always return the same
> value.