On 07/05/2024 14:30, Luca Fancellu wrote:
Hi Julien,
Hi Luca,
On 7 May 2024, at 14:20, Julien Grall <jul...@xen.org> wrote:
Hi Luca,
On 23/04/2024 09:25, Luca Fancellu wrote:
From: Penny Zheng <penny.zh...@arm.com>
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?
I looked at the code and, if I am not mistake, we are passing
PFN_DOWN(psize) to map_regions_p2mt. At the moment, it is unclear to me
why would psize be < PAGE_SIZE.
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.
Honestly, I can't remember why it wasn't commented.
Cheers,
--
Julien Grall
