Re: SVSM vTPM specification

["Daniel P. Smith" <dpsmith@apertussolutions.com>]

On 10/13/22 17:06, James Bottomley wrote:
> On Thu, 2022-10-13 at 16:54 -0400, Daniel P. Smith wrote:
>> Pardon the interjection.
>>
>> On 10/13/22 15:20, James Bottomley wrote:
>>> On Thu, 2022-10-13 at 13:54 -0500, Tom Lendacky wrote:
>>>> On 10/12/22 14:05, James Bottomley wrote:
>>>>> On Wed, 2022-10-12 at 18:33 +0100, Dr. David Alan Gilbert
>>>>> wrote:
>>>>>> * Tom Lendacky (thomas.lendacky@amd.com) wrote:
>>>> ...
>>>>> It is theoretically possible to emulate a CRB TPM with just a
>>>>> single
>>>>> communication page and an ACPI entry (the Linux CRB driver is
>>>>> ACPI
>>>>> only
>>>>> at this time and responds to the "MSFT0101" ACPI entry).
>>>>>
>>>>> The CRB device responds to a very compact MMIO region (0x30
>>>>> bytes
>>>>> long)
>>>>> described in the CRB spec:
>>>>>
>>>>> https://trustedcomputinggroup.org/resource/tpm-2-0-mobile-command-response-buffer-interface-specification/
>>
>> IMHO I would not use the mobile CRB, which was designed as a
>> doorbell solution for ARM where trapping a page was not possible.
>> Since it is possible to trap on page access, this makes it possible
>> to implement the PC Client MMIO interface and enables implementations
>> to provide the complete set of TPM capabilities, e.g. localities.
> 
> You mean the TIS interface:
> 
> https://trustedcomputinggroup.org/resource/pc-client-work-group-pc-client-specific-tpm-interface-specification-tis/
> 
> We discussed this at length with AMD ... which I think isn't captured
> in the email archives, unfortunately.  However, the bottom line is that
> TIS uses a FIFO approach to sending data through the MMIO page.  That's
> simply unscalable for pure emulation because it will result in 10-100x
> the number of write traps in the MMIO page as the CRB driver which uses
> a MMIO mailbox to trigger actions but passes the data via physical page
> addresses not FIFOs.

Yes I am referring to the TIS interface but FIFO is only one of the 
software interface defined in the spec. There is also the TIS CRB 
interface which I should enable a similar experience as the mobile CRB.

> So I think we could implement a reasonably performant CRB driver using
> page trapping emulation, but not a TIS one.  For an enlightened driver,
> a simple call interface seems to suffice.
> 
> James
> 
>