linux-kernel - Re: [PATCH v1 1/3] x86/tdx: Check for TDX partitioning during early TDX init

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Date:   Fri, 24 Nov 2023 12:04:56 +0100
From:   Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Michael Kelley <mhkelley58@...il.com>,
        Nikolay Borisov <nik.borisov@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org,
        Dexuan Cui <decui@...rosoft.com>, linux-hyperv@...r.kernel.org,
        stefan.bader@...onical.com, tim.gardner@...onical.com,
        roxana.nicolescu@...onical.com, cascardo@...onical.com,
        kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
        sashal@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH v1 1/3] x86/tdx: Check for TDX partitioning during early
 TDX init

On 24/11/2023 11:43, Kirill A. Shutemov wrote:
> On Fri, Nov 24, 2023 at 11:31:44AM +0100, Jeremi Piotrowski wrote:
>> On 23/11/2023 14:58, Kirill A. Shutemov wrote:
>>> On Wed, Nov 22, 2023 at 06:01:04PM +0100, Jeremi Piotrowski wrote:
>>>> Check for additional CPUID bits to identify TDX guests running with Trust
>>>> Domain (TD) partitioning enabled. TD partitioning is like nested virtualization
>>>> inside the Trust Domain so there is a L1 TD VM(M) and there can be L2 TD VM(s).
>>>>
>>>> In this arrangement we are not guaranteed that the TDX_CPUID_LEAF_ID is visible
>>>> to Linux running as an L2 TD VM. This is because a majority of TDX facilities
>>>> are controlled by the L1 VMM and the L2 TDX guest needs to use TD partitioning
>>>> aware mechanisms for what's left. So currently such guests do not have
>>>> X86_FEATURE_TDX_GUEST set.
>>>>
>>>> We want the kernel to have X86_FEATURE_TDX_GUEST set for all TDX guests so we
>>>> need to check these additional CPUID bits, but we skip further initialization
>>>> in the function as we aren't guaranteed access to TDX module calls.
>>>
>>> I don't follow. The idea of partitioning is that L2 OS can be
>>> unenlightened and have no idea if it runs indide of TD. But this patch
>>> tries to enumerate TDX anyway.
>>>
>>> Why?
>>>
>>
>> That's not the only idea of partitioning. Partitioning provides different privilege
>> levels within the TD, and unenlightened L2 OS can be made to work but are inefficient.
>> In our case Linux always runs enlightened (both with and without TD partitioning), and
>> uses TDX functionality where applicable (TDX vmcalls, PTE encryption bit).
> 
> What value L1 adds in this case? If L2 has to be enlightened just run the
> enlightened OS directly as L1 and ditch half-measures. I think you can
> gain some performance this way.
> 

It's primarily about the privilege separation, performance is a reason
one doesn't want to run unenlightened. The L1 makes the following possible:
- TPM emulation within the trust domain but isolated from the OS
- infrastructure interfaces for things like VM live migration
- support for Virtual Trust Levels[1], Virtual Secure Mode[2]

These provide a lot of value to users, it's not at all about half-measures.

[1]: https://lore.kernel.org/lkml/1681192532-15460-1-git-send-email-ssengar@linux.microsoft.com/
[2]: https://lore.kernel.org/lkml/20231108111806.92604-1-nsaenz@amazon.com/