Re: [PATCH v2] public/io/netif.h: make control ring hash protocol more general

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2016-02-16 at 14:17 +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Ian Campbell [mailto:ian.campbell@citrix.com]
> > Sent: 16 February 2016 14:13
> > To: Paul Durrant; xen-devel@lists.xenproject.org
> > Cc: Ian Jackson; Jan Beulich; Keir (Xen.org); Tim (Xen.org)
> > Subject: Re: [PATCH v2] public/io/netif.h: make control ring hash
> > protocol
> > more general
> > 
> > On Tue, 2016-02-16 at 14:02 +0000, Paul Durrant wrote:
> > > > -----Original Message-----
> > > [snip]
> > > > > 
> > > > > > >   *
> > > > > > > - * NETIF_CTRL_TYPE_SET_TOEPLITZ_MAPPING_ORDER
> > > > > > > - * ------------------------------------------
> > > > > > > + * NETIF_CTRL_TYPE_SET_HASH_MAPPING_ORDER
> > > > > > 
> > > > > > This one needs a similar "if the hash algorithm requires it"
> > > > > > wording
> > > > > > like the setting the key one had.
> > > > > > 
> > > > > 
> > > > > Why? Is there any point of doing hashing at all if the backend is
> > > > > not
> > > > > going to map it to a queue via a mapping table?
> > > > 
> > > > But will all hashing algorithms work via a table with a variable
> > > > order?
> > > > 
> > > 
> > > My view is that the algorithm used to generate the hash (which is
> > > after
> > > all just a number) and then mapping that hash to a queue via a table
> > > are
> > > pretty separate. Do you have an example in mind where these things
> > > are
> > > more intertwined? (Maybe my view is too simplistic).
> > 
> > I don't know of a specific example, but was just trying to generalise
> > along
> > the lines this was already heading in order to avoid future headaches
> > when
> > trying to add new (perhaps not yet invented) schemes, e.g. to
> > algorithms
> > with fixed numbers of queues, which support non-power of two table
> > sizes
> > or
> > which take the hash output mod N as the queue number without passing
> > via
> > a
> > table lookup phase etc.
> 
> I could change things to allow for a non power-of-two hash table now, so
> I'll do that so as not to rule it out. And with that, of course, you can
> provide a table to give a simple hash-mod-N mapping.

I was envisaging something the other way round i.e. a hash which hardcoded
that hash-mod-N mapping, i.e. where it would be an error to try and set
some other table or arguably to permit setting any table at all even if it
happened to be 1:1 (since making the b/e for such an algorithm check seems
like unnecessary overhead/complexity).

Ian.

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