linux-kernel - Re: [RFC PATCH v7 01/19] Add a new structure for skb buffer from external.

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Message-ID: <20100620115926.GA31849@gondor.apana.org.au>
Date:	Sun, 20 Jun 2010 21:59:26 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	"Xin, Xiaohui" <xiaohui.xin@...el.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"jdike@...ux.intel.com" <jdike@...ux.intel.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC PATCH v7 01/19] Add a new structure for skb buffer from
	external.

On Sun, Jun 20, 2010 at 02:47:19PM +0300, Michael S. Tsirkin wrote:
>
> Let's do this then.  So far the virtio spec avoided making layout
> assumptions, leaving guests lay out data as they see fit.
> Isn't it possible to keep supporting this with zero copy for hardware
> that can issue DMA at arbitrary addresses?

I think you're mistaken with respect to what is being proposed.
Raising 512 bytes isn't a hard constraint, it is merely an
optimisation for Intel NICs because their PS mode can produce
a head fragment of up to 512 bytes.

If the guest didn't allocate 512 bytes it wouldn't be the end of
the world, it'd just mean that we'd either copy whatever is in
the head fragment, or we waste 4096-X bytes of memory where X
is the number of bytes in the head.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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