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RE: [Xen-devel] [PATCH] [VTD] Separate VT-d page table fromP2M table

To: "Espen Skoglund" <espen.skoglund@xxxxxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] [VTD] Separate VT-d page table fromP2M table
From: "Han, Weidong" <weidong.han@xxxxxxxxx>
Date: Tue, 22 Apr 2008 23:02:40 +0800
Cc: Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, "Kay, Allen M" <allen.m.kay@xxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, "Xu, Anthony" <anthony.xu@xxxxxxxxx>
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Thread-topic: [Xen-devel] [PATCH] [VTD] Separate VT-d page table fromP2M table
Espen Skoglund wrote:
> [Weidong Han]
>> Keir Fraser wrote:
>>> On 22/4/08 13:17, "Han, Weidong" <weidong.han@xxxxxxxxx> wrote:
>>>>> Are you saying that the VT-d 2MB page format is different from
>>>>> the EPT 2MB page format? Or that VTd does not support 2MB pages?
>>>> Now VTd does not support 2MB pages.
>>> Then don't allow EPT 2MB mappings for domains which have passthru
>>> devices? 
>> No, EPT is 2MB, VT-d page table is 4KB.
> So EPT only supports 2MB mappings?  Not 4KB?  That doesn't sound
> right. 

No, EPT can support both 4KB and 2MB pages. 

> Another possible inompatibility: The VT-d chipset I'm using only
> allows 4-level page tables.  Not sure if similar restrictions might
> apply to EPT.  Also, in another project I worked on I found it
> advantageous to emulate superpages in the guest even if this was not
> supported by the VT hardware --- lower memory footprint, quicker table
> lookups.  Such optimizations might be another reason for separating
> the tables.
> That said, I would really disfavor separating the tables.  There are
> enough memory management structures as it is right now.  If the tables
> really, really, really need to be separated then don't make it a boot
> time option.  The capabilities of VT-d (and probably also EPT) is
> readily available at initialization time, and that's where the
> decision for sharing or not should be done.

I think the option for sharing or not will be a temporary thing. If
community considers shared table or separate table is useless, we can
remove the one easily.

Randy (Weidong)

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