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SubjectRe: [PATCH] efi: fdt: avoid FDT manipulation after ExitBootServices()
On 2/1/2017 10:45 AM, Ard Biesheuvel wrote:
> Some AArch64 UEFI implementations disable the MMU in ExitBootServices(),
> after which unaligned accesses to RAM are no longer supported.
> Commit abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the
> kernel") fixed an issue in the memory map handling of the stub FDT code,
> but inadvertently created an issue with such firmwares, by moving some
> of the FDT manipulation to after the invocation of ExitBootServices().
> Given that the stub's libfdt implementation uses the ordinary, accelerated
> string functions, which rely on hardware handling of unaligned accesses,
> manipulating the FDT with the MMU off may result in alignment faults.
> So fix the situation by moving the update_fdt_memmap() call into the
> callback function invoked by efi_exit_boot_services() right before it
> calls the ExitBootServices() UEFI service (which is arguably a better
> place for it anyway)
> Note that disabling the MMU in ExitBootServices() is not compliant with
> the UEFI spec, and carries great risk due to the fact that switching from
> cached to uncached memory accesses halfway through compiler generated code
> (i.e., involving a stack) can never be done in a way that is architecturally
> safe.
> Cc: <>
> Fixes: abfb7b686a3e ("efi/libstub/arm*: Pass latest memory map to the kernel")
> Tested-by: Riku Voipio <>
> Signed-off-by: Ard Biesheuvel <>

NACK, please. This causes a regression on my platform, in the form of
an assert in UEFI once ExitBootServices() is called, per initial
testing. I'll do more testing to determine why.

Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

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