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

[Stefan Berger]

On 12/13/2011 12:41 PM, Anthony Liguori wrote:
> On 12/13/2011 07:51 AM, Michael S. Tsirkin wrote:
> > On Tue, Dec 13, 2011 at 07:51:17AM -0500, Stefan Berger wrote:
> > > On 12/12/2011 11:51 PM, Paul Brook wrote:
> > > > > > > +tpm_passthrough="no"
> > > > > > Same as before, please probe for existence.
> > > > > We would be probing for /dev/tpm0. Is that really what we want 
> > > > > that this
> > > > > driver only gets compiled if /dev/tpm0 is (currently) available?
> > > > If what you say is true then this code should always be enabled.
> > > Michael Tsirkin previously requested that there be an option for the
> > > TPM passthrough driver to be selectively enabled since at least
> > > using /dev/tpm0 may not be what everybody wants. The passthrough
> > > driver at some point will also be able to use sockets to communicate
> > > with a TPM when a file descriptor is passed to Qemu, so maybe that
> > > changes then?
> > >
> > >
> > >     Stefan
> >
> > The passthrough as it is, is pretty easy to misuse.
> > This is a hardware problem, not software, and
> > I don't think it's fixable.
>
> Can you elaborate?  And can this be documented such that users are 
> aware of this.

From qemu-doc.html:

Some notes about using the host's TPM with the passthrough driver:

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.
> Regards,
>
> Anthony Liguori

Stefan