Am 18. Dezember 2023 19:12:11 MEZ schrieb Simon Glass sjg@chromium.org:
Hi Heinrich,
On Sat, 16 Dec 2023 at 12:04, Heinrich Schuchardt xypron.glpk@gmx.de wrote:
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.
- 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).
- 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.
- 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.
- 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:
- 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.
- 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.
- 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.
- 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().
- 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.
- 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:
- 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.
- Add memory attributes to UCLASS_RAM and use them in EFI, mapping to
the EFI_MEMORY_... attributes in struct efi_mem_desc.
- 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.
- 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.
- 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.
- 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.
Thank you for your thoughts on this.
It does not look like my write-up has helped at all with getting aligned on this. Do you have any other ideas?
Perhaps we could at least figure out the 'allocation lifetime' approach? It seems clear to me that we have allocations with short lifetimes (e.g. kernel & ramdisk allocations will remain in effect only when booting). Do you agree with that?
We agree that we want to have a unified memory management.
Memory management implies that memory allocations remain in effect until the memory is freed.
This must be true for all allocations whether it is for a kernel, a ramdisk, for loading a file, for an allocation by an EFI binary, or for anything else.
If boot commands like bootm allocate memory and free it after a failure, that time will be short.
If a file is loaded and never unloaded that memory allocation must stay until the OS takes over memory management. This is what is missing in U-Boot.
Best regards
Heinrich