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

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Date:   Thu, 23 Nov 2023 16:58:46 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.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 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?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov