Re: [PATCH v2] ethdev: document RSS default key and types

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

Hi Shahaf,

On Tue, Nov 13, 2018 at 06:39:04PM +0000, Shahaf Shuler wrote:
> Hi Adrien, 
> 
> Tuesday, November 13, 2018 7:15 PM, Adrien Mazarguil:
> > Subject: Re: [dpdk-dev] [PATCH v2] ethdev: document RSS default key and
> > types
> > 
> > Again a bit late to the party, please see below.
> > 
> > On Sun, Nov 11, 2018 at 09:35:22AM +0000, Ori Kam wrote:
> 
> [...]
> 
> > > > The setfault is the result of commit a4391f8bae ("app/testpmd: set
> > > > default RSS key as null").
> > > > Reverting this commit should fix the segfault but it also means
> > > > there is no way to set default key (key=NULL) with testpmd.
> > > > Need to check if this is only a testpmd limitation and not all
> > > > applications limitation.
> > > >
> > > > We should decide how an application can set default RSS without
> > > > knowing anything about keys.
> > > >
> > >
> > > I agree with Adrian that the main criteria should be the length.
> > > Maybe the set default RSS in testpmd should get new parameter.
> > 
> > Since [1] was reverted and we seem to agree that a zero key_len should
> > trigger a PMD-specific default key, this can already be requested with
> > testpmd by overriding key_len, e.g.:
> > 
> >  flow create 1 pattern eth / end actions rss key_len 0 / end
> > 
> > Using an empty string as the key would yield the same result but cannot be
> > expressed on the command line yet. Note that specifying a key automatically
> > overrides key_len, so key_len must be forced to 0 last to get PMD defaults:
> > 
> >  flow create 1 pattern eth / end actions rss key foo key_len 0 / end
> 
> I don't understand why we are backing up API claims with "how testpmd is implemented". The APIs should be correct, regardless of how testpmd is using them. 

This wasn't the intent, I mean, currently one cannot input something like
that to get a zero key length:

 flow create 1 pattern eth / end actions rss key "" / end

Because "" is interpreted literally. So the only way to request a zero key
length is by explicitly setting it through "key_len 0".

The API remains clear: a zero key length requests default behavior from the
PMD regardless of the key pointer, which doesn't *have* to be NULL, merely
undefined. Testpmd does exactly that.

> To this doc issue, 
> I don't understand on what cases it makes sense for application to have rss_key_len = 0 while rss_key != NULL. This is obviously in-consist input, and of course all PMD will just ignore the key.
> I think enforcing rss_key and rss_key_len to be NULL is a fair requirement from application, and it makes no confusion in the API inputs.

Then you need to define what happens when key_len != 0 and key == NULL, also
when key_len == 0 and key != NULL, none of which make sense currently.

There's no reason for the PMD to even look at the key pointer if key_len is
0. Only if nonzero, it *can* check for its validity however there's no
reason to, it's a programming error in the application if not the case.
assert() is more appropriate for such situations.

I agree there's a lack of documentation which must be addressed, my point is
that key_len is the only guarantee a PMD needs from the application.

> > Here key_len is set to testpmd's default size when parsing "rss", updated to
> > 3 when parsing "key foo" and updated once again when parsing "key_len 0".
> > 
> > Lastly, while it would make sense for testpmd to use 0 as the default value,
> > doing so yields inconsistent balancing results between vendors/devices as
> > they all come with a different key. Same reason as initializing the RSS types
> > field to the global rss_hf instead of 0.
> 
> 
> 
> > 
> > [1] "app/testpmd: revert setting default RSS"
> > 
> > https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fma
> > ils.dpdk.org%2Farchives%2Fdev%2F2018-
> > November%2F118786.html&data=02%7C01%7Cshahafs%40mellanox.co
> > m%7C0eecf3e9af4b4b6bc53108d6498ba2a8%7Ca652971c7d2e4d9ba6a4d1492
> > 56f461b%7C0%7C0%7C636777261425388073&sdata=Hu0iGr2xS%2FI%2FI
> > s5PtzCylMMft5w5TBmtd3GYppEKKcA%3D&reserved=0

-- 
Adrien Mazarguil
6WIND