NVIndexes in TPMs can operate in two modes:
1. Backed by TPM RAM. In this case they are only written to NVRAM on an
orderly TPM shutdown when the system goes down. (TPMA_NV_ORDERLY flag
is on)
2. Backed by TPM NVRAM. In this case the nvindex value is written to NVRAM
on every write, and things are not delayed until orderly shutdown.
Normally mode 1 sounds like the obvious choice for NvPCRs, which reset
to zero anyway at boot. However, things are more complicated since
real-life TPMs tend to have a lot less RAM than NVRAM (both are
constrained but RAM even more than NVRAM). Hence there's value in using
NVRAM right-away. However, writing to NVRAM all the time means wearing
it out (since NVRAM is more vulnerable to that).
So far we unconditionally went for mode 1, but ran into space
constraints of RAM due to that.
Let's improve things a bit, and use orderly mode for NvPCRs we expect to
write many times, and non-orderly mode for those we expect to write only
a small, fixed number of times at boot, and not anymore during runtime.
Right now, this is only the "hardware" NvPCR, which measures hw identity
at boot.
Hopefully, this stretches available resources a bit further.
This also makes sure if the flag was set differently on allocation as
we'd set now, we accept it and won't complain, to make upgrades safe.
Suggested by Andreas Fuchs.
55be5e1890
