Re: [Xen-devel] [PATCH net v2 1/3] xen-netback: remove pointless clause from if statement

[David Laight <David.Laight@xxxxxxxxxx>]

From: Paul Durrant
> > A reasonable high estimate for the number of slots required for a specific
> > message is 'frag_count + total_size/4096'.
> > So if that are that many slots free it is definitely ok to add the message.
> >
> 
> Hmm, that may work. By total_size, I assume you mean skb->len, so that 
> calculation is based on an
> overhead of 1 non-optimally packed slot per frag. There'd still need to be a 
> +1 for the GSO 'extra'
> though.

Except I meant '2 * frag_count + size/4096' :-(

You have to assume that every fragment starts at n*4096-1 (so need
at least two slots). A third slot is only needed for fragments
longer that 1+4096+2 - but an extra one is needed for every
4096 bytes after that.

        David




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