Re: [PATCH v3 4/4] platform/x86: intel_tdx_attest: Add TDX Guest attestation interface driver

[Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 4/20/22 11:57 PM, Isaku Yamahata wrote:
> On Wed, Apr 20, 2022 at 07:42:06PM -0700,
> Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
> 
>>
>>
>> On 4/20/22 5:11 PM, Kai Huang wrote:
>>> On Wed, 2022-04-20 at 16:45 -0700, Sathyanarayanan Kuppuswamy wrote:
>>>> If we want to support multiple GetQuote requests in parallel, then we
>>>> need some way to uniquely identify the GetQuote requests. So that when
>>>> we get completion notification, we can understand which request is
>>>> completed. This part is not mentioned/discussed in ABI spec. So we want
>>>> to serialize the requests for now.
>>>>
>>>
>>> Yes it's unfortunate that this part (whether concurrent GetQuote requests are
>>> supported by TDX architecture) is not explicitly mentioned in GHCI spec.  I am
>>> fine with only supporting GetQuote requests one by one.  AFAICT there's no
>>> request to support concurrent GetQuote requests anyway.  What concerns me is
>>> exactly how explain this.
>>>
>>> As I said, we have GET_QUOTE_IN_FLIGHT flag now.  Theoretically, you can queue
>>> multiple GetQuote requests, and when you receive the interrupt, you check which
>>> buffer has GET_QUOTE_IN_FLIGHT cleared.  That buffer is the one with Quote
>>> ready.  However I am not 100% sure whether above will always work.  Interrupt
>>> can get lost when there are multiple Quotes ready in multiple buffer in very
>>> short time period, etc?  Perhaps Isaku can provide more input here.
>>
>> Either supported or not, it should be mentioned in the GHCI spec. Currently,
>> there are no details related to it. If it is supported, the specification
>> should include the protocol to use.
>>
>> I will check with Isaku about it.
> 
> The spec says that TD can call multiple GetQuote requests in parallel.

Sorry, I missed the above content. Thanks for pointing out.

> 
>    TDG.VP.VMCALL<GetQuote> API allows one TD to issue multiple requests. It's
>    implementation specific that how many concurrent requests are allowed. The TD
>    should be able to handle TDG.VP.VMCALL_RETRY if it chooses to issue multiple
>    requests simultaneously

Do you know why we should handle VMCALL_RETRY case? IIUC, as per
above spec, if each request we send uses different GPA buffer, then we
should not even worry about checking for IN_FLIGHT status. right?

> 
> As Kai said, there is no requirement for multiple GetQuote in parallel, it's
> okay to support only single request at the same time.

For now I will leave it as single request at a time.

> 
> While the status is GET_QUOTE_IN_FLIGHT, VMM owns the shared GPA.  The
> attestation driver should wait for GET_QUOTE_IN_FLIGHT to be cleared before
> sending next request.

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer