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From: Ard Biesheuvel <ardb@kernel.org>
Date: Fri, 1 Mar 2024 18:09:53 +0100
Message-ID: <CAMj1kXFJwEUExy7+Snh3QHVn-ATj0C+sYje22Qmc+y=cCtAV7g@mail.gmail.com>
Subject: Re: [PATCH v7 2/9] x86/startup_64: Defer assignment of 5-level paging
 global variables
To: Borislav Petkov <bp@alien8.de>
Cc: Ard Biesheuvel <ardb+git@google.com>, linux-kernel@vger.kernel.org, 
	Kevin Loughlin <kevinloughlin@google.com>, Tom Lendacky <thomas.lendacky@amd.com>, 
	Dionna Glaze <dionnaglaze@google.com>, Thomas Gleixner <tglx@linutronix.de>, 
	Ingo Molnar <mingo@redhat.com>, Dave Hansen <dave.hansen@linux.intel.com>, 
	Andy Lutomirski <luto@kernel.org>, Brian Gerst <brgerst@gmail.com>
Content-Type: text/plain; charset="UTF-8"

On Fri, 1 Mar 2024 at 17:09, Borislav Petkov <bp@alien8.de> 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