Re: [PATCH V2] i386: Using 64-bit boot protocol for 64-bit linux kernel

[Daniel Kiper]

On Sat, Jul 23, 2022 at 07:20:43PM +0800, Wei Zhang wrote:
> From: Wei Zhang <zhangweilst@gmail.com>
>
> Currently GRUB boots linux with 32-bit protocol for 64 bit kernel.
> Thus if both GRUB and linux kernel are in 64-bit, we'll have to go
> through 64-bit grub -> 32-bit boot protocol -> 64-bit kernel
> transitions, and extra instructions have to be executed in the kernel.
>
> Since linux has long ago supported 64-bit boot protocol, we can take
> advantage of that to directly boot to 64-bit kernel.
>
> To do this, first we determine whether the kernel is 64-bit by
> xloadflags (since linux boot protocol 2.12), then we build the
> identity-mapped page table required by the 64-bit kernel, and that's
> it. The memory needed by the page table is allocated after the
> protected kernel image proper.
>
> So if we're in 32-bit GRUB to boot a 64-bit kernel, the transition
> will happen before handing over to the kernel, and if we're in 64-bit
> GRUB, we don't have to go down to 32-bit and back to 64-bit. The
> 32-bit kernel boot process will not be affected.
>
> Tested on my 64-bit machine and QEMU.

Could you tell us what target and platform you configure for the GRUB?
Or send us full GRUB configure command you use?

Daniel