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Re: [PATCH for-4.19? 1/2] xen/x86: account for max guest gfn and number of foreign mappings in the p2m
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 6 May 2024 17:34:59 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 06 May 2024 15:35:04 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 06.05.2024 17:33, Roger Pau Monné wrote:
> On Mon, May 06, 2024 at 12:07:33PM +0200, Jan Beulich wrote:
>> On 30.04.2024 18:58, Roger Pau Monne wrote:
>>> Keep track of the maximum gfn that has ever been populated into the p2m, and
>>> also account for the number of foreign mappings. Such information will be
>>> needed in order to remove foreign mappings during teardown for HVM guests.
>>
>> Is "needed" the right term? We could e.g. traverse the P2M tree (didn't look
>> at patch 2 yet as to how exactly you use these two new fields there), at
>> which
>> point we might get away without either or both of these extra statistics,
>> while at the same time also not needing to iterate over a gigantic range of
>> GFNs. Going from populated page tables would roughly match "max_gfn", with
>> the
>> benefit of certain removals of P2M entries then also shrinking the upper
>> bound.
>
> One note about traversing the p2m tree that I forgot to add earlier:
> AFAICT we would need one implementation for EPT and one for NPT, as I
> expect the different page-table format won't allow us to use the same
> code against both EPT and NPT page-tables (I really need to check).
Yes, that would be pretty much unavoidable, I agree.
Jan
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