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

["Han, Weidong" <weidong.han@xxxxxxxxx>]

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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