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Re: [Xen-devel] [RFC Patch] Support for making an E820 PCI hole in tools

To: Keir Fraser <keir@xxxxxxx>
Subject: Re: [Xen-devel] [RFC Patch] Support for making an E820 PCI hole in toolstack (xl + xm)
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Mon, 15 Nov 2010 11:32:38 -0800
Cc: stefano.stabellini@xxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, bruce.edge@xxxxxxxxx, gianni.tedesco@xxxxxxxxxx, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
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On 11/15/2010 10:41 AM, Keir Fraser wrote:
>> Oh, there was another reason that I so obvious that I completly forgot. DomU
>> has no idea where the host PCI hole starts. In most cases it is at 3GB (or
>> even
>> further up - 3.5GB), but a quick look for 'Allocating PCI resources starting
>> at' 
>> at Google shows that there are some that start at 1.2G.
> Hm, true. We could give you access to XENMEM_machine_memory_map? It's not
> really got any big secrets or privileged things in it. :-)
> Or, is there much disadvantage, to having a static really big PCI hole? Say
> starting at 1GB? The advantage of this would be the ability to hotplug PCI
> devices to a domU even across save/restore/migrate -- this may not work so
> well if you commit yourself to the hole size of the original host, and the
> restore/migrate target host has a bigger hole!

Well, the other question is whether the devices have to have the same
pfn as mfn within the hole.  We're emulating the PCI config space anyway
- couldn't we stick the passthrough PCI space at 3G regardless of where
it is on the real hardware?


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