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Date:   Wed, 22 Mar 2023 19:15:41 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Cc:     linux-kernel@vger.kernel.org,
        Brijesh Singh <brijesh.singh@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "Kalra, Ashish" <ashish.kalra@amd.com>,
        linux-crypto@vger.kernel.org,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Len Brown <lenb@kernel.org>, linux-acpi@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org
Subject: Re: [PATCH v3 0/8] Support ACPI PSP on Hyper-V
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On Wed, Mar 22, 2023 at 06:33:37PM +0100, Jeremi Piotrowski wrote:
> What this does is it allows a normal (non-SNP) VM to host confidential (SNP)
> VMs. I say "normal" but not every VM is going to be able to do this, it needs

If you say "non-SNP" VM then this sounds like purely for development.
Because I cannot see how you're going to give the confidentiality
guarantee to the SNP guests if the lower level is unencrypted, non-SNP
and so on...

> to be running on AMD hardware and configured to have access to
> VirtualizationExtensions, a "HardwareIsolation" capability, and given a number
> of "hardware isolated guests" that it is allowed to spawn. In practice this
> will result in the VM seeing a PSP device, SEV-SNP related CPUID
> leafs, and have access to additional memory management instructions
> (rmpadjust/psmash).  This allows the rest of the of KVM-SNP support to
> work.

So why don't you emulate the PSP in KVM instead of doing some BIOS hack?
And multiplex the access to it between all the parties needing it?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette