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Re: [PATCH RFCv2 14/15] xen/arm: mm: Rework setup_xenheap_mappings()


On 15/05/2021 00:51, Stefano Stabellini wrote:
On Sun, 25 Apr 2021, Julien Grall wrote:
From: Julien Grall <julien.grall@xxxxxxx>

A few issues have been reported with setup_xenheap_mappings() over the
last couple of years. The main ones are:
     - It will break on platform supporting more than 512GB of RAM
       because the memory allocated by the boot allocator is not yet
     - Aligning all the regions to 1GB may lead to unexpected result
       because we may alias non-cacheable region (such as device or reserved

map_pages_to_xen() was recently reworked to allow superpage mappings and
deal with the use of page-tables before they are mapped.

Most of the code in setup_xenheap_mappings() is now replaced with a
single call to map_pages_to_xen().

This also require to re-order the steps setup_mm() so the regions are
given to the boot allocator first and then we setup the xenheap

I know this is paranoia but doesn't this introduce a requirement on the
size of the first bank in bootinfo.mem.bank[] ?

It should be at least 8KB?

AFAIK, the current requirement is 4KB because of the 1GB mapping. Long term, it would be 8KB.

I know it is unlikely but it is theoretically possible to have a first
bank of just 1KB.

All the page allocators are working at the page granularity level. I am not entirely sure whether the current Xen would ignore the region or break.

Note that setup_xenheap_mappings() is taking a base MFN and a number of pages to map. So this doesn't look to be a new problem here.

For the 8KB requirement, we can look at first all the pages to the boot allocator and then do the mapping.

I think we should write the requirement down with an in-code comment?
Or better maybe we should write a message about it in the panic below?

I can write an in-code comment but I think writing down in the panic() would be wrong because there is no promise this is going to be the only reason it fails (imagine there is a bug in the code...).


Julien Grall



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