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
>
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