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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

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?

> 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, 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 :-)