Re: [PATCH v3 17/19] xen/arm64: mm: Add memory to the boot allocator first

[Julien Grall <julien@xen.org>]

Hi Stefano,

On 05/04/2022 22:50, Stefano Stabellini wrote:
>> +static void __init setup_mm(void)
>> +{
>> +    const struct meminfo *banks = &bootinfo.mem;
>> +    paddr_t ram_start = ~0;
>> +    paddr_t ram_end = 0;
>> +    paddr_t ram_size = 0;
>> +    unsigned int i;
>> +
>> +    init_pdx();
>> +
>> +    /*
>> +     * We need some memory to allocate the page-tables used for the xenheap
>> +     * mappings. But some regions may contain memory already allocated
>> +     * for other uses (e.g. modules, reserved-memory...).
>> +     *
>> +     * For simplify add all the free regions in the boot allocator.
>> +     */
> 
> We currently have:
> 
> BUG_ON(nr_bootmem_regions == (PAGE_SIZE / sizeof(struct bootmem_region)));

This has enough space for 256 distinct regions on arm64 (512 regions on 
arm32).

> 
> Do you think we should check for the limit in populate_boot_allocator?

This patch doesn't change the number of regions added to the boot 
allocator. So if we need to check the limit then I would rather deal 
separately (see more below).

> Or there is no need because it is unrealistic to reach it?
I can't say never because history told us on some UEFI systems, there 
will be a large number of regions exposed. I haven't heard anyone that 
would hit the BUG_ON().

The problem is what do we do if we hit the limit? We could ignore all 
the regions after. However, there are potentially a risk there would not 
be enough memory to cover the boot memory allocation (regions may be 
really small).

So if we ever hit the limit, then I think we should update the boot 
allocator.

Cheers,

-- 
Julien Grall