Proposal: U-Boot memory management

[Tom Rini]

On Fri, Dec 29, 2023 at 06:44:15PM +0100, Mark Kettenis wrote:
> > Date: Fri, 29 Dec 2023 11:17:44 -0500
> > From: Tom Rini <trini at konsulko.com>
> > 
> > On Fri, Dec 29, 2023 at 05:05:17PM +0100, Heinrich Schuchardt wrote:
> > > 
> > > 
> > > 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.
> > 
> > I very much do not want to change the API. There's untold numbers of
> > scripts out there and they should continue to work. I mentioned to Ilias
> > off list just now that I'm not against adding a command to add flags to
> > these areas, but I don't think it's worthwhile to prevent overwrites the
> > user did early. The biggest long running problem in this space was that
> > for 32bit ARM we couldn't know where the kernel BSS was going to be and
> > so would have ramdisk at the wrong spot and get partially eaten, and
> > this was hard to figure out. The current example is "ooops,
> > decompression buffer for Image.gz/etc is too close to other things"
> > which ends up failing nice and loudly, and in the future once this
> > proposal is done we can just dynamically find and use a spot, since
> > we'll have that ability finally.
> 
> In order to keep the existing interfaces we need lmb to keep track of
> (at least ) three different states.  I think of those as "free",
> "allocated" and "reserved".  The load command would "reserve" memory
> insteaf "allocate".  And it would pass a flag to lmb when reserving
> memory to indicate that reserving memory that is already reserved is
> ok.  Both "reserved" and "allocated" memory should show up as not free
> in the EFI memory map (probably as EfiLoaderData).

Yes, something like this is what I was getting at, thanks.

-- 
Tom
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