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Re: [PATCH] x86/pv: limit GDT and LDT mappings areas to max number of vCPUs
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 21 Nov 2024 17:03:21 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 21 Nov 2024 16:03:48 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 21.11.2024 16:56, Roger Pau Monné wrote:
> On Thu, Nov 21, 2024 at 12:12:18PM +0100, Roger Pau Monne wrote:
>> The allocation of the paging structures in the per-domain area for mapping
>> the
>> guest GDT and LDT can be limited to the maximum number of vCPUs the guest can
>> have. The maximum number of vCPUs is available at domain creation since
>> commit
>> 4737fa52ce86.
>>
>> Limiting to the actual number of vCPUs avoids wasting memory for paging
>> structures that will never be used. Current logic unconditionally uses 513
>> pages, one page for the L3, plus 512 L1 pages.
>
> This is not true, I was confused with the logic in
> create_perdomain_mapping(). When create_perdomain_mapping() is called
> with pl1tab == NULL and ppg == NULL it just allocates the L2, but not
> the L1 tables.
>
> So the purpose of the create_perdomain_mapping(d, GDT_LDT_VIRT_START,
> ...) in pv_domain_initialise() is even more dubious now - as it just
> allocates a page to use as L2. I'm tempted to just remove it if you
> agree, since I don't consider this useful. The allocation will
> already be done at vCPU initialization.
If it's done implicitly there, removing is likely fine. It feels like it may
have been necessary to do this explicitly earlier on.
Jan
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