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

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Date:	Wed, 27 Feb 2013 15:49:14 +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 Wed, 2013-02-27 at 07:39 +0000, ANNIE LI wrote:
> 
> On 2013-2-26 20:35, Wei Liu wrote:
> > 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 did not see the overlap you mentioned here in netback. Although 
> netback supports multi-page, netback->vif still uses single page if the 
> frontend only supports single page. Netfront and netback negotiate this 
> through xenstore in your 5/8 patch. The requests and response should not 
> have any overlap between netback and netfront. Am I missing something?
> 

I tried to dig up mail archive just now and realized that the bug report
was in private mail exchange with Konrad.

I don't really remember the details now since it is more than one year
old, but you can find trace in Konrad's tree, CS 5b4c3dd5b255. All I can
remember is that this bug was triggered by mixed old/new
frontend/backend.

I think this cap can be removed if we make all buffers in netfront
dynamically allocated.


Wei.

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