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Re: [PATCH 3/7] xen/p2m: put reference for superpage
- To: Julien Grall <julien@xxxxxxx>
- From: Luca Fancellu <Luca.Fancellu@xxxxxxx>
- Date: Tue, 7 May 2024 13:30:37 +0000
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Penny Zheng <Penny.Zheng@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Bertrand Marquis <Bertrand.Marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Delivery-date: Tue, 07 May 2024 13:31:14 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH 3/7] xen/p2m: put reference for superpage
Hi Julien,
> On 7 May 2024, at 14:20, Julien Grall <julien@xxxxxxx> wrote:
>
> Hi Luca,
>
> On 23/04/2024 09:25, Luca Fancellu wrote:
>> From: Penny Zheng <Penny.Zheng@xxxxxxx>
>> We are doing foreign memory mapping for static shared memory, and
>> there is a great possibility that it could be super mapped.
>
> Is this because we are mapping more than one page at the time? Can you point
> me to the code?
So, to be honest this patch was originally in Penny’s serie, my knowledge of
this side of the codebase
is very limited and so I pushed this one basically untouched.
From what I can see in the serie the mappings are made in
handle_shared_mem_bank, and map_regions_p2mt
is called for one page at the time (allocated through the function
allocate_domheap_memory (new function introduced in
the serie).
So is that the case that this patch is not needed?
>
>> But today, p2m_put_l3_page could not handle superpages.
>
> This was done on purpose. Xen is not preemptible and therefore we need to be
> cautious how much work is done within the p2m code.
>
> With the below proposal, for 1GB mapping, we may end up to call put_page() up
> to 512 * 512 = 262144 times. put_page() can free memory. This could be a very
> long operation.
>
> Have you benchmark how long it would take?
I did not, since its purpose was unclear to me and was not commented in the
last serie from Penny.
Cheers,
Luca
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