On Mon, 21 Aug 2023 at 20:10, Ard Biesheuvel <a...@kernel.org> wrote: > > On Thu, 3 Aug 2023 at 15:24, Ard Biesheuvel <a...@kernel.org> wrote: > > > > The x86_64 Linux kernel can be booted in 32-bit mode, in which case the > > startup code creates a set of preliminary page tables that map the first > > 1GiB of physical memory 1:1, and enables paging. This is a prerequisite > > for 64-bit execution, and can therefore only be implemented in 32-bit > > code. > > > > The x86_64 Linux kernel can also be booted in 64-bit mode directly: this > > implies that paging is already enabled, and it is the responsibility of > > the bootloader to ensure that the the active page tables cover the > > entire loaded image, including its BSS space, the size of which is > > described in the image's setup header. > > > > Given that the EFI spec mandates execution in long mode for x86_64, and > > stipulates that all system memory is mapped 1:1, the Linux/x86 > > requirements for 64-bit entry can be met trivially when booting on > > x86_64 via EFI. So enter via the 64-bit entrypoint in this case. > > > > This involves inspecting the xloadflags field in the setup header to > > check whether the 64-bit entrypoint is supported. This field was > > introduced in Linux version v3.8 (early 2013) > > > > This change ensures that all EFI firmware tables and other assets passed > > by the firmware or bootloader in memory remain mapped and accessible > > throughout the early startup code. (Note that Linux's 32-bit startup > > code creates multiple shadow mappings of the first 1GiB of physical > > memory up to the 4 GiB mark so anything that resides there becomes > > inaccessible until the 64-bit startup code replaces the preliminary > > mappings with more accurate ones) > > > > Avoiding the drop out of long mode will also be needed to support > > upcoming CPU designs that no longer implement 32-bit mode at all (as > > recently announced by Intel [0]) > > > > [0] > > https://www.intel.com/content/www/us/en/developer/articles/technical/envisioning-future-simplified-architecture.html > > > > Cc: Daniel Kiper <daniel.ki...@oracle.com> > > Cc: Julian Andres Klode <julian.kl...@canonical.com> > > Signed-off-by: Ard Biesheuvel <a...@kernel.org> > > Ping? >
Ping? This should preferably be part of the release so we avoid failing back to 32-bit entry for configurations that may have been using the handover protocol in the past. _______________________________________________ Grub-devel mailing list Grub-devel@gnu.org https://lists.gnu.org/mailman/listinfo/grub-devel
