remove UEFI reserved regions from the linear mapping

Quoted 19 lines · Mark Rutland

Quoted 7 lines · Ard Biesheuvel

This is yet another approach to solving the issues around removing RAM regions known to UEFI from the linear mapping while preserving the record of the fact that these regions are backed by memory.

The previous approach added a memblock flag called MEMBLOCK_NOMAP to keep track of RAM regions that should be removed from the linear mapping.

Quoted 6 lines

The primary motivation for the new approach is the observation that there is only a single use case that requires this, which is acpi_os_ioremap(). Since ACPI implies UEFI on arm64 platforms, and since acpi_os_ioremap() uses page_is_ram() internally (which is a __weak generic function), we can simply reimplement page_is_ram() to take the UEFI memory map into account if we are booted via UEFI.

Just to check, is the above the only reason for this new approach? Or were there other issues with the MEMBLOCK_NOMAP approach other than the diffstat?

Quoted 2 lines

I quite liked the MEMBLOCK_NOMAP approach as it looked reusable.

I think the MEMBLOCK_NOMAP approach is sound, but it is harder to prove that there are no corner cases that behave incorrectly.

Quoted 3 lines

I take it there aren't any lurking instances of page_is_ram() used to test if something exists in the linear mapping?

Well, first of all, the linear mapping only covers lowmem, so that in itself would not be a portable use. In general, pfn_valid() would be the correct test for that (possibly combined with PageHighmem())

page_is_ram() and the 'System RAM' iomem region are so poorly defined or documented that we may be better off just removing it in the first place and replace it with something meaningful.

Quoted 7 lines

Quoted 3 lines

Patch #1 slightly reorders the UEFI runtime services initialization routines so that the EFI_MEMMAP flag is only set if the permanent mapping of the UEFI memory map is in place.

This also means that the memory map is mapped even with EFI runtime support disabled, but I guess that's not a big problem.

Yes. You need that anyway if you are going to rely on it even when the runtime services are disabled.

Quoted 4 lines

As a side thought, it would be nice if we could memremap_ro the system table and memory map in future to prevent potential corruption, given they have fixed VAs and are always mapped.

I agree, and I already have some local patches using early_memremap_ro() for the EFI init code.

memremap_ro() does not actually exist yet, but I intend to propose MEMREMAP_RO and MEMREMAP_NX flags to Dan Williams's memremap() work once I get around to it.