Re: [RFC] Proposed x86 shadow paging changes

["Tu Dinh" <ngoc-tu.dinh@xxxxxxxxxx>]

On 12/06/2025 18:48, Teddy Astie wrote:
> Le 12/06/2025 à 14:12, Jan Beulich a écrit :
>> On 12.06.2025 13:45, Teddy Astie wrote:
>>> In Xen x86, there is the shadow paging that could be used in some cases
>>> (notably with HVM), and is used when HAP is not available. It may also
>>> be used in very specific cases for PV guests.
>>>
>>> With the vast majority of hardware supporting HAP (through EPT or NPT),
>>> it's rarely used in practice for HVM, with most downstream projects
>>> (XenServer, XCP-ng, QubesOS) are compiling-out its support by default.
>>> It also performs very significantly worse than HAP.
>>
>> Whenever this argument was made, iirc it was in particular George (who
>> was involved in the original work aiui) who kept pointing that such a
>> statement is not generally true, for numbers being workload dependent.
>>
>
> I am a bit curious on specific workloads where shadow paging performs
> better than hap. Especially considering that shadow paging doesn't
> support PCID and the existence of ASIDs.
>

This is quite common in big-memory workloads where you don't touch the
memory mappings much. With HAP you can lose 5-7% perf due to the impact
on TLB misses.


Ngoc Tu Dinh | Vates XCP-ng Developer

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