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From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCHv3] virtio-spec: 64 bit features, used/avail event, fixes
Date: Wed, 3 Aug 2011 19:29:55 +0300
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Cc: Rusty Russell <rusty@rustcorp.com.au>,
	virtualization@lists.linux-foundation.org,
	habanero@linux.vnet.ibm.com, Shirley Ma <mashirle@us.ibm.com>,
	Krishna Kumar2 <krkumar2@in.ibm.com>, kvm@vger.kernel.org,
	steved@us.ibm.com, Tom Lendacky <tahm@linux.vnet.ibm.com>,
	borntraeger@de.ibm.com, avi@redhat.com,
	bryanv@daemoninthecloset.org
To: Gerd Hoffmann <kraxel@redhat.com>
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On Wed, Aug 03, 2011 at 06:15:33PM +0200, Gerd Hoffmann wrote:
>   Hi,
> 
> >E.g. with 32 bridges, and 32 devices behind each one,
> >the available 64K space gets us only 64 bytes per device.
> 
> 15 bridges (with io window enabled) max, the smallest io window you
> can assign to a bridge is 4k,

Hmm true, I missed that. So with 32 devs we get 256 bytes per device.
We can still get low on that space when using
multifunction devices though (there could be up to 256 functions behind
a bridge, which only leaves 16 bytes per device).

BTW, this limitation will be a problem for pci express
devices.

> and you need some space for the
> devices on the root bus ...
> 
> cheers,
>   Gerd