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Date:   Fri, 22 Jul 2022 14:18:58 -0700
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Subject: Re: [PATCH v8 5/5] x86/tdx: Add Quote generation support
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To:     Dave Hansen <dave.hansen@intel.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kai Huang <kai.huang@intel.com>,
        Wander Lairson Costa <wander@redhat.com>,
        marcelo.cerri@canonical.com, tim.gardner@canonical.com,
        khalid.elmously@canonical.com, philip.cox@canonical.com,
        linux-kernel@vger.kernel.org,
        "Nakajima, Jun" <jun.nakajima@intel.com>
References: <20220609025220.2615197-1-sathyanarayanan.kuppuswamy@linux.intel.com>
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 <18578c5a-7a35-ab20-467c-80141b0410a8@intel.com>
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
In-Reply-To: <18578c5a-7a35-ab20-467c-80141b0410a8@intel.com>
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+ Jun

On 7/22/22 12:13 PM, Dave Hansen wrote:
> On 7/22/22 12:05, Isaku Yamahata wrote:
>>> So, the quote portion of this is basically a bidirectional blob sender.
>>>  It's to send a blob between guest userspace to host userspace.
>>>
>>> Do we *REALLY* need specific driver functionality for this?  For
>>> instance, is there no existing virtio device that can send blobs back
>>> and forth?
>> It's virtio-vsock.  If virtio-vsock is available, the communication works.
>> However, some users would like to disable virtio-vsock on their environment for
>> some reasons.  Even virtio at all.  Especially for confidential computing use
>> case.  It's their choice.  It can't be assumed that virtio is available.
>>
>> The goal is VMM-agnostic (but TDX-specific) interface for that.
> 
> You're basically saying that every confidential computing technology
> should have its own host user <-> guest kernel <-> guest user ABI.
> That's insanity.  If we do this, we need *one* interface that says "talk
> to the hypervisor" that's common for all hypervisors and hardware
> vendors, or at least more than *one*.
> 
> We don't need a way to talk to hypervisors for Intel systems and another
> for AMD and yet another on whatever.

For cases where your platform does not want to support or enable the generic
interface (like vsock), isn't it better to have a fallback approach? I am not
saying we should have such an ABI for all cases. But attestation is a must-have
feature for the TDX guest, and we want to support it in all TD guest platforms.
I think the GHCI ABI is added to meet this requirement.

Jun/Isaku, if you are aware of the exact requirement for this hypercall, please
share it. Also let us know your comments on this topic.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer