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

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

On 11/18/2016 09:43 PM, Jarkko Sakkinen wrote:
> 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.

Thanks Jarkko, Yeah, Sure, I will write it as different non-generic call.
Otherwise, the function works good.
Also, I am thinking now I can write "multi-bank support for extend" on 
top of master branch itself.  Any issues with that ?

Thanks & Regards,
    - Nayna

>
> /Jarkko
>