Re: [Qemu-devel] [PATCH V13 6/7] Introduce --enable-tpm-passthrough configure option

[Paul Brook <paul@codesourcery.com>]

> The TPM device accessed by the passthrough driver must not be used by
> any other application on the host.
> 
> Since the host's firmware (BIOS/UEFI) has already initialized the TPM,
> the VM's firmware (BIOS/UEFI) will not be able to initialize the TPM
> again and may therefore not show a TPM-specific menu that would
> otherwise allow the user to configure the TPM, e.g., allow the user to
> enable/disable or activate/deactivate the TPM. Further, if TPM ownership
> is released from within a VM then the host's TPM will get disabled and
> deactivated. To enable and activate the TPM again afterwards, the host
> has to be rebooted and the user is required to enter the firmware's menu
> to enable and activate the TPM. If the TPM is left disabled and/or
> deactivated most TPM commands will fail.

Presumably the same is true of any other application that has access to 
/dev/tpm0?

This doesn't sound any different to any other kind of device passthrough (e.g. 
USB).  If the host is also talking to the device then it's likely to get 
confused.  If the guest tells the device to do something suicidal then it'll 
most likely die.

Complicated of cource by the fact that the whole point of the TPM is you don't 
really trust yourself, so the usual failure mode is a complete nervous 
breakdown.  But that comes with the territory.

Paul