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

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Date:   Thu, 7 Dec 2023 18:21:33 +0100
From:   Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "mhkelley58@...il.com" <mhkelley58@...il.com>,
        "Cui, Dexuan" <decui@...rosoft.com>
Cc:     "cascardo@...onical.com" <cascardo@...onical.com>,
        "tim.gardner@...onical.com" <tim.gardner@...onical.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "roxana.nicolescu@...onical.com" <roxana.nicolescu@...onical.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "stefan.bader@...onical.com" <stefan.bader@...onical.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "sashal@...nel.org" <sashal@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v1 1/3] x86/tdx: Check for TDX partitioning during early
 TDX init

On 07/12/2023 13:58, Huang, Kai wrote:
>>
>> That's how it currently works - all the enlightenments are in hypervisor/paravisor
>> specific code in arch/x86/hyperv and drivers/hv and the vm is not marked with
>> X86_FEATURE_TDX_GUEST.
> 
> And I believe there's a reason that the VM is not marked as TDX guest.
Yes, as Elena said:
"""
OK, so in your case it is a decision of L1 VMM not to set the TDX_CPUID_LEAF_ID
to reflect that it is a tdx guest and it is on purpose because you want to 
drop into a special tdx guest, i.e. partitioned guest. 
"""
TDX does not provide a means to let the partitioned guest know that it needs to
cooperate with the paravisor (e.g. because TDVMCALLs are routed to L0) so this is
exposed in a paravisor specific way (cpuids in patch 1).

> 
>>
>> But without X86_FEATURE_TDX_GUEST userspace has no unified way to discover that an
>> environment is protected by TDX and also the VM gets classified as "AMD SEV" in dmesg.
>> This is due to CC_ATTR_GUEST_MEM_ENCRYPT being set but X86_FEATURE_TDX_GUEST not.
> 
> Can you provide more information about what does _userspace_ do here?

I gave one usecase in a different email. A workload scheduler like Kubernetes might want to
place a workload in a confidential environment, and needs a way to determine that a VM is
TDX protected (or SNP protected) to make that placement decision.

> 
> What's the difference if it sees a TDX guest or a normal non-coco guest in
> /proc/cpuinfo?
> 
> Looks the whole purpose of this series is to make userspace happy by advertising
> TDX guest to /proc/cpuinfo.  But if we do that we will have bad side-effect in
> the kernel so that we need to do things in your patch 2/3.
> 

Yes, exactly. It's unifying the two approaches so that userspace doesn't have to
care.

> That doesn't seem very convincing.

Why not? 
The whole point of the kernel is to provide a unified interface to userspace and
abstract away these small differences. Yes it requires some kernel code to do,
thats not a reason to force every userspace to implement its own logic. This is
what the flags in /proc/cpuinfo are for.

> Is there any other way that userspace can
> utilize, e.g., any HV hypervisor/paravisor specific attributes that are exposed
> to userspace?
> 

There are no HV hyper-/para-visor attributes exposed to userspace directly, but
userspace can poke at the same cpuid bits as in patch 1 to make this determination.
Not great for confidential computing adoption.