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

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Date: Fri, 1 Mar 2024 13:13:47 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Borislav Petkov <bp@...en8.de>, Ard Biesheuvel <ardb@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org,
 Kevin Loughlin <kevinloughlin@...gle.com>,
 Dionna Glaze <dionnaglaze@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Andy Lutomirski <luto@...nel.org>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH v7 2/9] x86/startup_64: Defer assignment of 5-level paging
 global variables

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@...en8.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
> 
> 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.
>