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

[Tom Lendacky]

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.