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

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Date:	Tue, 31 Jan 2012 13:32:50 +0000
From:	Wei Liu <wei.liu2@...rix.com>
To:	Jan Beulich <JBeulich@...e.com>
CC:	<wei.liu2@...rix.com>, 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 Tue, 2012-01-31 at 13:24 +0000, Jan Beulich wrote:
> >>> 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.
> 

IMHO, this is not a critical part of the code, so failing this one thus
making all other pieces not workable is very strange.

> 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).
> 

I understand your concern. If the changes required will not make this
series longer and involves major changes in block interface, I'm happy
to refactor the xenbus interface.


Wei.

> Jan
> 


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