On Fri, Nov 18, 2016 at 05:42:01PM +0530, Nayna wrote: > > > 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 for pointing that. I think you got it right and I had some wrong assumptions about 'moreData'. Here's what I propose. Do a non-generic function just for getting CAP_PCRS. You could call it tpm2_get_pcr_allocation() as you don't want or rather need to handle all the bells and whistles in that TPM command. It makes a lot more sense now than having one-size-for-all function. /Jarkko