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

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Message-Id: <4F27F9960200007800070228@nat28.tlf.novell.com>
Date:	Tue, 31 Jan 2012 13:24:22 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Wei Liu" <wei.liu2@...rix.com>
Cc:	"Ian Campbell" <Ian.Campbell@...rix.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 31.01.12 at 12:09, Wei Liu <wei.liu2@...rix.com> 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.

There's so many other allocations, why would you worry about this
one.

But of course you can undo what a recent change did, and then
subsequently someone else will have to clean up again after you.
I'm just asking to follow good programming practices and write
re-usable code where potential for re-use is obvious (after all,
multi-page block interface patches have been floating around for
much longer than yours for the net interface).

Jan

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