RE: [Xen-devel] [PATCH] [VTD] Separate VT-d page table from P2M table

[Espen Skoglund <espen.skoglund@xxxxxxxxxxxxx>]

[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.

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.

        eSk


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