about context gap

Jarkko Sakkinen jarkko.sakkinen at linux.intel.com
Mon Sep 18 18:02:50 UTC 2017

On Sat, Sep 16, 2017 at 08:36:29AM -0700, Jarkko Sakkinen wrote:
> Hi
> While sitting at the airport I started to think a bit about the context
> gap issue.
> I think first thing that would make sense would be to have a 64-bit
> shadow count for every TPM space i.e.
> atomic_long_t tpm2_ctx_cnt;
> struct tpm2_space {
> 	/* ... */
> 	u64 ctx_cnt;
> };
> For every create and load of a session you will read the global count
> and increase it. This way we have get a well defined order.
> With this organization we could for example put tpm spaces to a
> red-black tree and refresh the oldest tpm space (just first thought
> that came to mind).
> PS. While doing LPC and LSS slides I found error from the TPM 2.0
> commands specification. TPM2_ContextSave can never emit
> TPM2_RC_CONTEXT_GAP albeit the specification says so.
> /Jarkko

Other than context gap it would probably make sense to estimate TPM
capacity in order to make implementation more intelligent. We could
measure memory capacity of a TPM in the driver initialization by filling
it with fixed size dummy objects.

TPM2_GetCapability gives stuff that cannot be really trusted as spec
compliant TPM HW could just return always '1' for max.

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