Re: [QUESTION] reservation of pool allocations performed during PEI

[Laszlo Ersek <lersek@redhat.com>]

On 01/20/17 17:42, Ard Biesheuvel wrote:
> On 20 January 2017 at 16:38, Laszlo Ersek <lersek@redhat.com> wrote:
>> On 01/20/17 17:05, Ard Biesheuvel wrote:
>>> After a recent change to the AArch64 page table code, the root table
>>> of the page tables is allocated using AllocatePool() rather than
>>> AllocatePages() if its size is much smaller than a page. E.g., when
>>> using 40 bits of translation, the root table only takes up 16 bytes
>>>
>>> However, what I have noticed is that pool allocations made during PEI
>>> are listed as available memory in the EFI memory map (using memmap in
>>> the UEFI Shell). Is this expected? Is it part of the contract that
>>> AllocatePool() allocations are lost when entering DXE?
>>
>> Pool allocations in PEI are satisfied with HOBs (and therefore the pool
>> allocation sizes are limited to ~64KB).
>>
>> In addition, soon after permanent PEI RAM is installed, objects from the
>> temporary SEC/PEI heap are moved there (this is called "temporary RAM
>> migration"). This includes the migration of the full HOB list, including
>> those that were used to satisfy pool allocations prior to RAM migration.
>>
> 
> So how is this supposed to work for code that holds a pointer to such
> a pool allocation?

It's not.

Against such HOBs, don't hold a pointer, hold a grudge. :)

> 
>> If you want to allocate memory in PEI that is to survive in-place into
>> DXE and later, I can think of two ways:
>>
>> - Call AllocatePages. This will only work after permanent PEI RAM has
>> been installed (so you might want to make the PEIM performing the call
>> dependent on gEfiPeiMemoryDiscoveredPpiGuid, with a DEPEX). The
>> allocation will be carved out of the permanent PEI RAM.
>>
> 
> The same code calls AllocatePages for the subsequent translation
> levels, i.e., when using 40 bits of translation, there are 4 levels,
> where all but the top level are full pages. So I could simply replace
> AllocatePool with AllocaPages in this case,

Yes, I think so.

> which would effectively
> revert my 'improvement' to this code to use a pool allocation for the
> root level.
> 
>> - Allocate a region (a whole multiple of pages) outside of the permanent
>> PEI RAM, but in a spot that will later on be backed by system memory
>> (due to a system memory resource descriptor HOB produced in PEI, or due
>> to a GCD memory space addition during DXE). The way to perform this kind
>> of allocation is simply to produce a memory allocation HOB, covering the
>> range in question. This works even before the installation of permanent
>> PEI RAM.
>>
>> ... I hope I remembered most of this stuff right.
>>
> 
> Thanks for the lesson :-)
> 

You certainly got what you paid for! ;)