Re: Discussion of Xenheap problems on AArch64

[Julien Grall <julien@xxxxxxx>]

On 21/04/2021 07:28, Henry Wang wrote:
> Hi,
Hi Henry,

> We are trying to implement the static memory allocation on AArch64. Part of
> this feature is the reserved heap memory allocation, where a specific range of
> memory is reserved only for heap. In the development process, we found a
> pitfall in current AArch64 setup_xenheap_mappings() function.
>
> According to a previous discussion in community
> https://lore.kernel.org/xen-devel/20190216134456.10681-1-peng.fan@xxxxxxx/,
> on AArch64, bootmem is initialized after setup_xenheap_mappings(),
> setup_xenheap_mappings() may try to allocate memory before memory has been
> handed over to the boot allocator. If the reserved heap memory allocation is
> introduced, either of below 2 cases will trigger a crash:
>
> 1. If the reserved heap memory is at the end of the memory block list and the
> gap between reserved and unreserved memory is bigger than 512GB, when we setup
> mappings from the beginning of the memory block list, we will get OOM caused
> by lack of pages in boot allocator. This is because the memory that is reserved
> for heap has not been mapped and added to the boot allocator.
>
> 2. If we add the memory that is reserved for heap to boot allocator first, and
> then setup mappings for banks in the memory block list, we may get a page which
> has not been setup mapping, causing a data abort.
There are a few issues with setup_xenheap_mappings(). I have been 
reworking the code on my spare time and started to upstream bits of it. 
A PoC can be found here:
https://xenbits.xen.org/gitweb/?p=people/julieng/xen-unstable.git;a=shortlog;h=refs/heads/pt/dev

Cheers,

--
Julien Grall