Proposal: U-Boot memory management

Fri Dec 29 17:05:17 CET 2023

Am 29. Dezember 2023 16:43:07 MEZ schrieb Tom Rini <trini at konsulko.com>:
>On Fri, Dec 29, 2023 at 05:36:09AM +0000, Simon Glass wrote:
>> Hi,
>> 
>> On Sat, Dec 16, 2023 at 6:01 PM Simon Glass <sjg at chromium.org> 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()).
>> >
>> > 3. U-Boot doesn't really map arch-specific memory attributes (e.g.
>> > armv8's struct mm_region) to EFI ones.
>> >
>> > 4. EFI duplicates some code from bootm, some of which relates to
>> > memory allocation (e.g. FDT fixup).
>> >
>> > 5. EFI code is used even if EFI is never used to boot
>> >
>> > 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.
>> 
>> 7. We need to support doing an allocation when a file is loaded (to
>> ensure files do not overlap), without making it too difficult to load
>> multiple files to the same place, if desired.
>> 
>> >
>> >
>> > 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.
>> >
>> > 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).
>> >
>> > ** 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.
>> >
>> >
>> > 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()
>> >
>> > 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.
>> >
>> > 4. Avoid setting up EFI memory at the start of U-Boot. Do it only when
>> > booting. This looks to require very little effort.
>> >
>> > 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.
>> >
>> > 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.
>> 
>> 7. Add a flag to the 'load' command:
>> 
>> -m <type> - make an lmb allocation for the file
>>    <type> is the image type to use (kernel, ramdisk, flat_dt)
>> 
>> any existing allocation for that type will be automatically freed
>> first. If <type> is "none" then no freeing is possible: any loaded
>> images just stack up in lmb.
>> 
>> Add an 'lmb' (or memblock) command to allow listing and clearing allocations.
>
>I would really not like to change the user interface and instead simply
>handle this with flags to whatever mark/allocation function is called.
>You can always overwrite things that are brought in to memory, you
>cannot overwrite U-Boot or our internals. Optionally noting that some
>previous load to memory has been at least partially overwritten could be
>helpful, if it's not too much extra logic.
>

In most use cases users load exactly one file at  each address. An unload command would be the cleanest way for a user to indicate that he wants to reuse the memory.

Best regards

Heinrich