Proposal: U-Boot memory management

[Tom Rini]

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.

-- 
Tom
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