netdev - Re: [Xen-devel] [RFC PATCH V3 12/16] netback: multi-page ring support

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Message-ID: <1328008350.26983.355.camel@zakaz.uk.xensource.com>
Date:	Tue, 31 Jan 2012 11:12:30 +0000
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	"Wei Liu (Intern)" <wei.liu2@...rix.com>
CC:	Jan Beulich <JBeulich@...e.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [Xen-devel] [RFC PATCH V3 12/16] netback: multi-page ring
 support

On Tue, 2012-01-31 at 11:09 +0000, Wei Liu (Intern) wrote:
> On Tue, 2012-01-31 at 09:01 +0000, Jan Beulich wrote:
> > >>> On 30.01.12 at 18:10, Wei Liu <wei.liu2@...rix.com> wrote:
> > > On Mon, 2012-01-30 at 16:35 +0000, Jan Beulich wrote:
> > >> >>> On 30.01.12 at 15:45, Wei Liu <wei.liu2@...rix.com> wrote:
> > >> > -int xenvif_map_frontend_rings(struct xenvif *vif,
> > >> > -			      grant_ref_t tx_ring_ref,
> > >> > -			      grant_ref_t rx_ring_ref)
> > >> > +int xenvif_map_frontend_rings(struct xen_comms *comms,
> > >> > +			      int domid,
> > >> > +			      unsigned long ring_ref[],
> > >> > +			      unsigned int  ring_ref_count)
> > >> >  {
> > >> > -	void *addr;
> > >> > -	struct xen_netif_tx_sring *txs;
> > >> > -	struct xen_netif_rx_sring *rxs;
> > >> > -
> > >> > -	int err = -ENOMEM;
> > >> > +	struct gnttab_map_grant_ref op[NETBK_MAX_RING_PAGES];
> > >> > +	unsigned int i;
> > >> > +	int err = 0;
> > >> >  
> > >> > -	err = xenbus_map_ring_valloc(xenvif_to_xenbus_device(vif),
> > >> > -				     tx_ring_ref, &addr);
> > >> 
> > >> Any reason why you don't just extend this function (in a prerequisite
> > >> patch) rather than open coding a common utility function (twice) here,
> > >> so that other backends (blkback!) can benefit later as well.
> > >> 
> > >> Jan
> > >> 
> > > 
> > > I'm mainly focusing on netback stuffs, so the code is slightly coupled
> > > with netback -- NETBK_MAX_RING_PAGES.
> > > 
> > > To extend xenbus_map_ring_valloc and make more generic, it requires
> > > setting a global maximum page number limits on rings, I think it will
> > > require further investigation and code refactor -- which I have no time
> > > to attend to at the moment. :-/
> > 
> > Why? You can simply pass in the number of pages, there's no need
> > for a global maximum.
> > 
> 
> I mean the gnttab_map_gran_ref array, it is statically allocated at the
> moment. Of course we can make it dynamically allocated, but why bother
> taking the risk of allocation failure.

You can do
	struct gnttab_map_grant_ref op[nr_pages];
and the compiler will do the right thing with the on-stack data
structure. In the kernel you'd need to be a bit careful about the size
of pages first but that should be all.

Ian.



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