linux-kernel - Re: [RFC 00/14] AMD: Add Secure AVIC Guest Support

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Message-ID: <fd5ea224-5c69-4e51-81d7-5a259bfec668@amd.com>
Date: Tue, 29 Oct 2024 20:58:55 +0530
From: Neeraj Upadhyay <Neeraj.Upadhyay@....com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
 tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
 Thomas.Lendacky@....com, nikunj@....com, Santosh.Shukla@....com,
 Vasant.Hegde@....com, Suravee.Suthikulpanit@....com, David.Kaplan@....com,
 x86@...nel.org, hpa@...or.com, peterz@...radead.org, seanjc@...gle.com,
 pbonzini@...hat.com, kvm@...r.kernel.org
Subject: Re: [RFC 00/14] AMD: Add Secure AVIC Guest Support

>>> Have you tested the case when the target kernel doesn't support SAVIC and
>>> tries to use a new interrupt vector on the boot CPU? I think it will
>>> break.
>>>
>>
>> For a VM launched with VMSA feature containing Secure AVIC, the target
>> kernel also is required to support Secure AVIC. Otherwise, guest bootup
>> would fail. I will capture this information in the documentation.
>> So, as far as I understand, SAVIC kernel kexecing into a non-SAVIC kernel
>> is not a valid use case.
> 
> Hm. I thought if SAVIC is not enabled by the guest the guest would boot
> without the secure feature, no?
> 

Actually no. The guest VM which is launched by VMM with Secure AVIC enabled
would have SecureAVIC reported in sev_status MSR. Secure AVIC is part of
SNP_FEATURES_IMPL_REQ  and guest boot would terminate due to snp_get_unsupported_features()
check in arch/x86/boot/compressed/sev.c if secure avic is not enabled (having said that,
I need to update config rules to select CONFIG_AMD_SECURE_AVIC if CONFIG_AMD_MEM_ENCRYPT
is enabled).

- Neeraj