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

Kai Huang
2022-04-21T10:33:21Z

On Wed, 2022-04-20 at 23:57 -0700, 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.
>
> 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
>
> As Kai said, there is no requirement for multiple GetQuote in parallel, it's
> okay to support only single request at the same 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.

Sorry I missed this in the spec. Then as I mentioned above, TD should check
which buffer has GET_QUOTE_IN_FLIGHT bit cleared to determine which GetQuote
request is done? I guess this is the only way.

Anyway, supporting single request only is fine to me. Just needs some
explanation in comments or commit message.

--
Thanks,
-Kai