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Message-ID: <1d25221c-eaab-0f97-83aa-8b4fbe3a53ed@linux.microsoft.com>
Date:   Wed, 22 Mar 2023 18:33:37 +0100
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Subject: Re: [PATCH v3 0/8] Support ACPI PSP on Hyper-V
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To:     Borislav Petkov <bp@alien8.de>
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
References: <20230320191956.1354602-1-jpiotrowski@linux.microsoft.com>
 <20230322154655.GDZBsi75f6LnQStxSp@fat_crate.local>
From:   Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
In-Reply-To: <20230322154655.GDZBsi75f6LnQStxSp@fat_crate.local>
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On 22/03/2023 16:46, Borislav Petkov wrote:
> On Mon, Mar 20, 2023 at 07:19:48PM +0000, Jeremi Piotrowski wrote:
>> This series is a prerequisite for nested SNP-host support on Hyper-V
> 
> I'm curious: what in the *world* is a sensible use case for doing this
> thing at all?
> 

This is actually not as crazy as it sounds.

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
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 instead of taking a bare-metal AMD server with 128 CPUs to run confidential
workloads you'll be able to provision an Azure VM with say 8 CPUs and run up to
8 SNP guests nested inside it.

It's also useful for development, I participate in the kata-containers project
where we're doing confidential-containers related work, and having access to
test VMs to run SNP guests is going to make things much easier.

If you're interested, I posted the other half of the patches required some time
back: https://lore.kernel.org/lkml/20230213103402.1189285-1-jpiotrowski@linux.microsoft.com/#t

Jeremi