linux-kernel - Re: [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory acceptance

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Message-ID: <96762dec-616b-b906-02b3-e006d43a8535@amd.com>
Date: Thu, 17 Apr 2025 11:21:30 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>, Ard Biesheuvel <ardb+git@...gle.com>,
 linux-efi@...r.kernel.org, x86@...nel.org, mingo@...nel.org,
 linux-kernel@...r.kernel.org,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Dionna Amalie Glaze <dionnaglaze@...gle.com>,
 Kevin Loughlin <kevinloughlin@...gle.com>
Subject: Re: [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory
 acceptance

On 4/17/25 11:14, Ard Biesheuvel wrote:
> On Thu, 17 Apr 2025 at 18:08, Tom Lendacky <thomas.lendacky@....com> wrote:
>>
>> On 4/11/25 14:00, Ard Biesheuvel wrote:
>>> On Fri, 11 Apr 2025 at 20:40, Borislav Petkov <bp@...en8.de> wrote:
>>>>
>>>> On Thu, Apr 10, 2025 at 03:28:51PM +0200, Ard Biesheuvel wrote:
>>>>> From: Ard Biesheuvel <ardb@...nel.org>
>>>>>
>>>>> Communicating with the hypervisor using the shared GHCB page requires
>>>>> clearing the C bit in the mapping of that page. When executing in the
>>>>> context of the EFI boot services, the page tables are owned by the
>>>>> firmware, and this manipulation is not possible.
>>>>>
>>>>> So switch to a different API for accepting memory in SEV-SNP guests, one
>>>>
>>>> That being the GHCB MSR protocol, it seems.
>>>>
>>>
>>> Yes.
>>>
>>>> And since Tom co-developed, I guess we wanna do that.
>>>>
>>>> But then how much slower do we become?
>>>>
>>>
>>> Non-EFI stub boot will become slower if the memory that is used to
>>> decompress the kernel has not been accepted yet. But given how heavily
>>> SEV-SNP depends on EFI boot, this typically only happens on kexec, as
>>> that is the only boot path that goes through the traditional
>>> decompressor.
>>
>> Some quick testing showed no significant differences in kexec booting
>> and testing shows everything seems to be good.
>>
> 
> Thanks.
> 
>> But, in testing with non-2M sized memory (e.g. a guest with 4097M of
>> memory) and without the change to how SNP is detected before
>> sev_enable() is called, we hit the error path in arch_accept_memory() in
>> arch/x86/boot/compressed/mem.c and the boot crashes.
>>
> 
> Right. So this is because sev_snp_enabled() is based on sev_status,
> which has not been set yet at this point, right?

Correct.

> 
> And for the record, could you please indicate whether you are ok with
> the co-developed-by/signed-off-by credits on this patch (and
> subsequent revisions)?

Yep, I'm fine with that.

Thanks,
Tom