On 12/16/23 19:01, Simon Glass wrote:
Hi,
This records my thoughts after a discussion with Ilias & Heinrich re
memory allocation in U-Boot.
1. malloc()
malloc() is used for programmatic memory allocation. It allows memory
to be freed. It is not designed for very large allocations (e.g. a
10MB kernel or 100MB ramdisk).
2. lmb
lmb is used for large blocks of memory, such as those needed for a
kernel or ramdisk. Allocation is only transitory, for the purposes of
loading some images and booting. If the boot fails, then all lmb
allocations go away.
lmb is set up by getting all available memory and then removing what
is used by U-Boot (code, data, malloc() space, etc.)
lmb reservations have a few flags so that areas of memory can be
provided with attributes
There are some corner cases...e.g. loading a file does an lmb
allocation but only for the purpose of avoiding a file being loaded
over U-Boot code/data. The allocation is dropped immediately after the
file is loaded. Within the bootm command, or when using standard boot,
this would be fairly easy to solve.
Linux has renamed lmb to memblock. We should consider doing the same.
3. EFI
EFI has its own memory-allocation tables.
Like lmb, EFI is able to deal with large allocations. But via a 'pool'
function it can also do smaller allocations similar to malloc(),
although each one uses at least 4KB at present.
EFI allocations do not go away when a boot fails.
With EFI it is possible to add allocations post facto, in which case
they are added to the allocation table just as if the memory was
allocated with EFI to begin with.
The EFI allocations and the lmb allocations use the same memory, so in
principle could conflict.
EFI allocations are sometimes used to allocate internal U-Boot data as
well, if needed by the EFI app. For example, while efi_image_parse()
uses malloc(), efi_var_mem.c uses EFI allocations since the code runs
in the app context and may need to access the memory after U-Boot has
exited. Also efi_smbios.c uses allocate_pages() and then adds a new
mapping as well.
EFI memory has attributes, including what the memory is used for (to
some degree of granularity). See enum efi_memory_type and struct
efi_mem_desc. In the latter there are also attribute flags - whether
memory is cacheable, etc.
EFI also has the x86 idea of 'conventional' memory, meaning (I
believe) that below 4GB that isn't reserved for the hardware/system.
This is meaningless, or at least confusing, on ARM systems.
4. reservations
It is perhaps worth mentioning a fourth method of memory management,
where U-Boot reserves chunks of memory before relocation (in
board_init_f.c), e.g. for the framebuffer, U-Boot code, the malloc()
region, etc.
Problems
—-------
There are no urgent problems, but here are some things that could be improved:
1. EFI should attach most of its data structures to driver model. This
work has started, with the partition support, but more effort would
help. This would make it easier to see which memory is related to
devices and which is separate.
2. Some drivers do EFI reservations today, whether EFI is used for
booting or not (e.g. rockchip video rk_vop_probe()).
Hello Simon,
thank you for summarizing our discussion.
Some U-Boot drivers including rockchip video inform the EFI sub-system
that memory is reserved.
Furthermore drivers like arch/arm/mach-bcm283x/reset.c exist that are
still used after ExitBootServices. mmio addresses have to be updated
when Linux creates its virtual memory map. Currently this is done via
efi_add_runtime_mmio(). A more UEFI style method would be to register an
event handler for ExitBootServices() and use ConvertPointer() in the
event handler.
3. U-Boot doesn't really map arch-specific memory attributes (e.g.
armv8's struct mm_region) to EFI ones.
U-Boot fails to set up RWX properties. E.g. the region where a FIT image
is loaded should not be executable.
4. EFI duplicates some code from bootm, some of which relates to
memory allocation (e.g. FDT fixup).
Fixup code is not duplicated but invoked via image_setup_libfdt().
5. EFI code is used even if EFI is never used to boot
* Only a minimum initialization of the EFI sub-system happens in
efi_init_early().
* Some EFI code is called when probing block devices because we wanted
the EFI and the dm part to be integrated.
* The rest of the initialization in efi_init_obj_list() is only invoked
if an EFI command is invoked.
6. EFI allocations can result in the same memory being used as has
already been allocated by lmb. Users may load files which overwrite
memory allocated by EFI.
The most worrisome issue is that EFI may allocate memory where U-Boot
has loaded files like initrd as the EFI sub-system is never informed
which memory is used for files.
Loading files should not be possible without creating a memory
reservation that becomes visible to the EFI sub-system.
Lifetime
--------
We have three different memory allocators with different purposes. Can
we unify them a little?
Within U-Boot:
- malloc() space lives forever
- lmb lives while setting out images for booting
- EFI (mostly) lives while booting an EFI app
In practice, EFI is set up early in U-Boot. Some of this is necessary,
some not. EFI allocations stay around forever. This works OK since
large allocations are normally not done in EFI, so memory isn't really
consumed to any great degree by the boot process.
U-Boot can load EFI drivers which stay resident in memory after the
efi_main() method has returned to U-Boot. The next EFI application then
may use the driver. Therefore is essential that the EFI subsystem has
access to a valid memory model at all times.
What happens to EFI allocations if the app returns? They are still
present, in case another app is run. This seems fine.
API
–--
Can we unify some APIs?
It should be possible to use lmb for large EFI memory allocations, so
long as they are only needed for booting. We effectively do this
today, since EFI does not manage the arrangement of loaded images in
memory. for the most part.
It would not make sense to use EFI allocation to replace lmb and
malloc(), of course.
Could we use a common (lower-level) API for allocation, used by both
lmb and EFI? They do have some similarities. However they have
different lifetime constraints (EFI allocations are never dropped,
unlikely lmb).
The way lmb is used is a deficiency of U-Boot. E.g. you can load an
initrd that overwrites the previously loaded kernel and then try to boot.
What we need is a common memory management library where allocations are
never dropped and which is used by all file loads.
** Overall, it seems that the existence of memory allocation in
boot-time services has created confusion. Memory allocation is
muddled, with both U-Boot code and boot-time services calling the same
memory allocator. This just has not been clearly thought out.
We have to implement what the UEFI specification requires. Some
boot-time services must allocate memory via AllocatePool() or
AllocatePages() because that memory is handed out to the caller of an
API function and it is the callers obligation to free the memory via
FreePool() or FreePages().
Proposal
—-------
Here are some ideas:
1. For video, use the driver model API to locate the video regions, or
block off the entire framebuffer memory, for all devices as a whole.
Use efi_add_memory_map()
When video memory is located higher than the stack the EFI sub-system
will not make use of the memory.
2. Add memory attributes to UCLASS_RAM and use them in EFI, mapping to
the EFI_MEMORY_... attributes in struct efi_mem_desc.
3. Add all EFI reservations just before booting the app, as we do with
devicetree fixup. With this model, malloc() and lmb are used for all
allocation. Then efi_add_memory_map() is called for each region in
turn just before booting. Memory attributes are dealt with above. The
type (enum efi_memory_type) can be determined simply by the data
structure stored in it, as is done today. For example, SMBIOS tables
can use EFI_ACPI_RECLAIM_MEMORY. Very few types are used and EFI code
understands the meaning of each.
This would require a permanent storage of the reservations. Keep it
easy, unify the memory management and make it persistent.
4. Avoid setting up EFI memory at the start of U-Boot. Do it only when
booting. This looks to require very little effort.
There is no such thing as EFI memory. We only have one physical memory
that we have to keep track of.
It is not possible to set up the EFI memory map if you don't keep track
of all memory allocations including all file loads over the whole
lifetime of U-Boot.
5. Avoid calling efi_allocate_pages() and efi_allocate_pool() outside
boot-time services. This solves the problem 6. If memory is needed by
an app, allocate it with malloc() and see 3. There are only two
efi_allocate_pages() (smbios and efi_runtime). There are more calls of
efi_allocate_pool(), but most of these seem easy to fix up. For
example, efi_init_event_log() allocates a buffer, but this can be
allocated in normal malloc() space or in a bloblist.
If we have a unified memory allocation layer, efi_allocate_pages() and
efi_allocate_pool() will be implemented by calls into that layer and
issue 6) will vanish.
6. Don't worry too much about whether EFI will be used for booting.
The cost is likely not that great: use bootstage to measure it as is
done for driver model. Try to minmise the cost of its tables,
particularly for execution time, but otherwise just rely on the
ability to disable EFI_LOADER.
The time intensive part of having EFI enabled is scanning file systems
for boot files and capsules.
Best regards
Heinrich
