(overlapping) lmb allocations during boot

Michael Walle michael at walle.cc
Sun Oct 31 16:19:56 CET 2021

Am 2021-10-31 13:51, schrieb Mark Kettenis:
>> Date: Sun, 31 Oct 2021 13:04:40 +0100
>> From: Michael Walle <michael at walle.cc>
>> Am 2021-10-31 12:44, schrieb Mark Kettenis:
>> >> Date: Sun, 31 Oct 2021 11:43:38 +0100
>> >> From: Michael Walle <michael at walle.cc>
>> >>
>> >> Hi,
>> >>
>> >> I sometimes see a corrupted initrd during kernel boot on my board
>> >> (kontron_sl28). Debugging showed that in this case the spin table
>> >> for the secondary CPUs overlaps with the lmb initrd allocations
>> >> (initrd_high is set).
>> >>
>> >> I had a look at how the fdt and initrd are relocated (if enabled). In
>> >> summary, they use lmb_alloc() which will first add all available
>> >> memory
>> >> and then carve out reserved regions. There are calls for arch
>> >> (arch_lmb_reserve()) and board (board_lmb_reserve()) callbacks. The
>> >> interesting thing is that, there is also code to carve out any
>> >> reserved regions which were added to the fdt earlier
>> >> (boot_fdt_add_mem_rsv_regions()). The problem here is, that the DT
>> >> fixups,
>> >> which might add the reserved regions are called just before jumping to
>> >> linux. Thus both the allocation for the fdt (if fdt_high is set) and
>> >> the ramdisk (if initrd_high is set) will ignore any reserved memory
>> >> regions.
>> >>
>> >> Unfortunately, I don't see any good way to fix this. You'd need all
>> >> the
>> >> DT fixups before we can initialize the lmb. Also, I don't know if this
>> >> will affect any other areas; probably I'm the only one, who reserves
>> >> an
>> >> area which is outside of the u-boot code and data segment.
>> >>
>> >> A hackish way would be to carve out the spin_table code in
>> >> board_lmb_reserve(). But meh..
>> >
>> > The spin table is embedded in the u-boot binary itself isn't it?  But
>> > the memory occupied by the u-boot should already be reserved...
>> Yes it is. As long as it doen't overlap with the 64k EFI code page.
>> See below.
> Yes, I saw the comment.  Which confuses me.  The overlap itself
> shouldn't be a problem since both the spin table and the EFI runtime
> code page should be preserved.  But I guess that the overlap causes
> problems later on because attempting to reserve overlapping areas
> fails?

TBH I don't remember the specifics anymore, my first try to fix this
problem was here [1]. But the conclusion was that reserving 64k for
the EFI alone would increase the binary size a lot. Thus Heinrich
suggested to relocate the spin table code.

>> > Unless CONFIG_EFI_LOADER is defined.  Then it relocates the spin table
>> > to memory allocated using efi_allocate_pages().  But that function
>> > only looks at the EFI memory map to figure out what memory is
>> > available.  So I suspect that it might hand out the same memory as
>> > lmb_alloc().  It all looks a bit broken to me...
>> Yes, that is actually my code ;) The kontron_sl28 is the only
>> board which uses spin tables as far as I know. It doesn't support
>> PSCI; at least if you don't load a bl31 TF-A. Therefore, for SMP
>> it uses spin tables. The relocation code work arounds a problem
>> with the reserved EFI code, see [1].
>> And yes, it actually is broken. But so might be every code which
>> is using the efi_allocate_pages(), no? LMB isn't global, but is
>> just initialized at different places. Like before a linux kernel
>> is booted or when you load a file (?). And everytime the whole
>> memory is added, and then different regions are carved out (see
>> above).
> Right.  So it mostly works because U-Boot either ends up using a code
> path where it uses LMB or a path where it ends up using the EFI memory
> map, but never both.  Except if it hits your layerscape code.
> I think the right way to solve this is to allocate the memory for the
> spin table through some other means and use efi_add_memory_map() to
> reserve that memory like what is done in board/raspberrypi/rpi/rpi.c.

Mh. I don't think this will work, just because lmb is quite stupid
and has just the three steps to carve out memory:
  (1) arch_lmb_reserve()
  (2) board_lmb_reserve()
  (3) boot_fdt_add_mem_rsv_regions()

(1) will carve out u-boot code and data (2) is usually not used and
(3) doesn't really work IMHO, because the reserved regions are added
much later in the bootm flow.

So what would "other means" be? I could allocate the memory somehow,
but how can I communicate that to lmb, so it would be excluded there.

> Or don't relocate the spin table and fix the issue with overlapping
> reserves.

Which hadn't much appeal ;) And doesn't fix (fundamental) lmb problem.



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