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

From: Tom Lendacky
Date: Fri Mar 01 2024 - 14:14:03 EST


On 3/1/24 11:33, Borislav Petkov wrote:
On Fri, Mar 01, 2024 at 06:09:53PM +0100, Ard Biesheuvel wrote:
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

Nice, that might come in handy for other testing too.

Be aware that additional work will need to be done in OVMF to support 5-level paging for SEV VMs.

Initial SEV implementation happened when there wasn't a page table library and so SEV support had to roll it's own page table modifications. A page table library has since been created and 5-level support was added, but the SEV code hasn't been converted over to use the new library, yet.

Thanks,
Tom


Thx.