netdev - Re: [PATCH 6/8] netfront: multi-page ring support

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Message-ID: <1361882139.2109.52.camel@zion.uk.xensource.com>
Date:	Tue, 26 Feb 2013 12:35:39 +0000
From:	Wei Liu <wei.liu2@...rix.com>
To:	ANNIE LI <annie.li@...cle.com>
CC:	<wei.liu2@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>
Subject: Re: [PATCH 6/8] netfront: multi-page ring support

On Tue, 2013-02-26 at 06:52 +0000, ANNIE LI wrote:
> 
> On 2013-2-16 0:00, Wei Liu wrote:
> > Signed-off-by: Wei Liu<wei.liu2@...rix.com>
> > ---
> >   drivers/net/xen-netfront.c |  246 +++++++++++++++++++++++++++++++-------------
> >   1 file changed, 174 insertions(+), 72 deletions(-)
> >
> > diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> > index 8bd75a1..de73a71 100644
> > --- a/drivers/net/xen-netfront.c
> > +++ b/drivers/net/xen-netfront.c
> > @@ -67,9 +67,19 @@ struct netfront_cb {
> >
> >   #define GRANT_INVALID_REF   0
> >
> > -#define NET_TX_RING_SIZE __CONST_RING_SIZE(xen_netif_tx, PAGE_SIZE)
> > -#define NET_RX_RING_SIZE __CONST_RING_SIZE(xen_netif_rx, PAGE_SIZE)
> > -#define TX_MAX_TARGET min_t(int, NET_TX_RING_SIZE, 256)
> > +#define XENNET_MAX_RING_PAGE_ORDER XENBUS_MAX_RING_PAGE_ORDER
> > +#define XENNET_MAX_RING_PAGES      (1U<<  XENNET_MAX_RING_PAGE_ORDER)
> > +
> > +
> > +#define NET_TX_RING_SIZE(_nr_pages)                  \
> > +     __CONST_RING_SIZE(xen_netif_tx, PAGE_SIZE * (_nr_pages))
> > +#define NET_RX_RING_SIZE(_nr_pages)                  \
> > +     __CONST_RING_SIZE(xen_netif_rx, PAGE_SIZE * (_nr_pages))
> > +
> > +#define XENNET_MAX_TX_RING_SIZE NET_TX_RING_SIZE(XENNET_MAX_RING_PAGES)
> > +#define XENNET_MAX_RX_RING_SIZE NET_RX_RING_SIZE(XENNET_MAX_RING_PAGES)
> > +
> > +#define TX_MAX_TARGET min_t(int, NET_TX_RING_SIZE(1), 256)
> 
> Not using multi-page ring here?
> In xennet_create_dev, gnttab_alloc_grant_references allocates
> TX_MAX_TARGET number of grant reference for tx. In
> xennet_release_tx_bufs, NET_TX_RING_SIZE(np->tx_ring_pages) numbers of
> grants are processed. And NET_RX_RING_SIZE(np->tx_ring_pages) is totally
> different from TX_MAX_TARGET if np->rx_ring_pages is not 1. Although
> skb_entry_is_link helps to not release invalid grants, lots of null loop
> seems unnecessary. I think TX_MAX_TARGET should be changed into some
> variableconnected with np->tx_ring_pages. Or you intended to use one
> page ring here?
> 

Looking back my history, this limitation was introduced because if we
have a multi-page backend and single page frontend, the backend skb
processing could overlap.

I agree with you that this limit should be variable, but as we still use
M:N model, the safe option is to cap this limit to 1 page.

Another option is to check validity of skbs before processing them. I
will look into that as well.

The same reason applies to the RX ring as well.


Wei.

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