Re: [Xen-devel] [PATCH v7 2/2] public/io/netif.h: document control ring and toeplitz hashing

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Mon, 2016-01-18 at 09:35 -0700, Jan Beulich wrote:
> > > > On 18.01.16 at 17:07, <ian.campbell@xxxxxxxxxx> wrote:
> > On Tue, 2016-01-12 at 09:58 +0000, Paul Durrant wrote:
> > > + * NETIF_CTRL_TYPE_SET_TOEPLITZ_MAPPING
> > > + * ------------------------------------
> > > + *
> > > + * This is sent by the frontend to set the content of the table
> > > mapping
> > > + * toeplitz hash value to queue number. The backend should calculate
> > > the
> > > + * hash from the packet header, use it as an index into the table
> > > (modulo
> > > + * the size of the table) and then steer the packet to the queue
> > > number
> > > + * found at that index.
> > > + *
> > > + * Request:
> > > + *
> > > + *ÂÂtypeÂÂÂÂ= NETIF_CTRL_TYPE_SET_TOEPLITZ_MAPPING
> > > + *ÂÂdata[0] = grant reference of page containing the mapping (sub-
> > > )table
> > > + *ÂÂÂÂÂÂÂÂÂÂÂÂ(assumed to start at beginning of grant)
> > > + *ÂÂdata[1] = size of (sub-)table in entries
> > > + *ÂÂdata[2] = offset, in entries, of sub-table within overall table
> > 
> > Adding data[2] seems reasonable to me, but if you wanted to avoid
> > adding it
> > then saying data[1][8:0] == size and data[1][31:9] == offset would
> > allow a
> > size of 512 (biggest possible in a single gref) and 8.3M for the
> > offset.
> 
> I may be lacking some context here, so please ignore if it reads
> like nonsense, but is the 9-bit width bound to a page size of 4k,
> or an inherent property of the Toeplitz algorithm?

The table entries are 8 bytes, so in that respect yes I suppose it is
related to the page size.

Paul thought it best not to do this any.

>  In the former
> case I think we should try to avoid putting further limitations to
> 4k pages into any interface. (And as an aside, 512 is also not
> representable in 9 bits, but this can clearly be addressed quite
> easily.)

Indeed, looks like I miscounted.

Ian.

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