Re: [PATCH v7 2/9] x86/startup_64: Defer assignment of 5-level paging global variables

[Ard Biesheuvel]

On Fri, 1 Mar 2024 at 17:09, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Fri, Mar 01, 2024 at 11:01:33AM +0100, Ard Biesheuvel wrote:
> > The scenario that I have not managed to test is entering from EFI with
> > 5 levels of paging enabled, and switching back to 4 levels (which
> > should work regardless of CONFIG_X86_5LEVEL). However, no firmware in
> > existence actually supports that today, and I am pretty sure that this
> > code has never been tested under those conditions to begin with. (OVMF
> > patches are under review atm to allow 5-level paging to be enabled in
> > the firmware)
>
> Aha.
>

I've built a debug OVMF image using the latest version of the series,
and put it at [0]

Run like this

qemu-system-x86_64 -M q35 \
  -cpu qemu64,+la57 -smp 4 \
  -bios OVMF-5level.fd \
  -kernel arch/x86/boot/bzImage \
  -append console=ttyS0\ earlyprintk=ttyS0 \
  -vga none -nographic -m 1g \
  -initrd <initrd.img>

and you will get loads of DEBUG output from the firmware first, and
then boot into Linux. (initrd can be omitted)

Right before entering, it will print

CpuDxe: 5-Level Paging = 1

which confirms that the firmware is running with 5 levels of paging.

I've confirmed that this boots happily with this series applied,
including when using 'no5lvl' on the command line, or when disabling
CONFIG_X86_5LEVEL [confirmed by inspecting
/sys/kernel/debug/page_tables/kernel].


[0] http://files.workofard.com/OVMF-5level.fd.gz