Re: [tpmdd-devel] [PATCH RFC 2/2] tpm: refactor tpm2_get_tpm_pt to tpm2_getcap_cmd

[Nayna <nayna@linux.vnet.ibm.com>]

On 11/17/2016 11:12 PM, Jarkko Sakkinen wrote:
> On Thu, Nov 17, 2016 at 05:20:36PM +0530, Nayna wrote:
>
>> I tested this for capability TPM2_CAP_PCRS. It seems TPM2_CAP_PCRS
>> capability always returns full PCR allocation, and more_data as 0, So, I
>> think the idea of looping over based on more_data may not work for this
>> capability.
>
> You can always request one value at a time until there's no more.
>
> If you request N values, depending on the hardware, the hardware returns
> to you anything from 1 to N values. If you implement a function that
> requests N values in the command, you *must* handle the case where
> moreData is 1 even if the hardware you are testing that never happens.
>
> That's the reason why I would start with a function that you request one
> property of one capability and optimize it in future if it doesn't scale
> for some workload.
>
> Do you have a workload where it doesn't scale?

Thanks Jarkko for explaining in detail.

If I understood correctly, the idea is to request for one property at a 
time, and if we need multiple properties, then to request for each of 
them in a loop. In case of TPM2_CAP_PCRS, property is always zero. This 
is how I am calling getcap_cmd for TPM2_CAP_PCRS.

tpm2_getcap_cmd(chip, TPM2_CAP_PCRS, 0, &cap_data, "get active pcr 
banks");

Output :

[   17.081665] tpm: cap id to receive value is 2
[   17.081666] tpm: TPM2_CAP_COMMANDS: more data 1
[   17.081667] tpm: 2
[   17.081668] tpm: tpm2_get_active_banks  -------> cap is TPM2_CAP_PCRS
[   17.171665] tpm: cap id to receive value is 5
[   17.171666] tpm: TPM2_CAP_PCRS: more data 0 ---> more data is zero.
[   17.171666] tpm: TPM2_CAP_PCRS: more data 0
[   17.171667] tpm: count pcr banks is 2 ------> count of active pcr 
banks information returned

more_data is always zero here, so am not sure how to handle more_data in 
this case ?
Since property_id is always zero, I am not able to request for one 
property at a time.
and response_buffer returns the details for both active banks.

This is the expected behavior defined in TCG 2.0 Part 3 Commands 
Specification (Section 30.2.1):

"TPM_CAP_PCRS – Returns the current allocation of PCR in a 
TPML_PCR_SELECTION. The property parameter shall be zero. The TPM will 
always respond to this command with the full PCR allocation and moreData 
will be NO."

Please let me know, if I am missing something.

Thanks & Regards,
    - Nayna


>
> /Jarkko
>