Re: [Qemu-devel] [PATCH v5 1/4] Provide support for the CUSE TPM

[Daniel P. Berrange]

On Wed, Jan 20, 2016 at 05:58:02PM +0200, Michael S. Tsirkin wrote:
> On Wed, Jan 20, 2016 at 10:36:41AM -0500, Stefan Berger wrote:
> > "Michael S. Tsirkin" <address@hidden> wrote on 01/20/2016 10:20:58 AM:
> > 
> > > From: "Michael S. Tsirkin" <address@hidden>
> > 
> > > >
> > > > The CUSE TPM and associated tools can be found here:
> > > >
> > > > https://github.com/stefanberger/swtpm
> > > >
> > > > (please use the latest version)
> > > >
> > > > To use the external CUSE TPM, the CUSE TPM should be started as follows:
> > > >
> > > > # terminate previously started CUSE TPM
> > > > /usr/bin/swtpm_ioctl -s /dev/vtpm-test
> > > >
> > > > # start CUSE TPM
> > > > /usr/bin/swtpm_cuse -n vtpm-test
> > > >
> > > > QEMU can then be started using the following parameters:
> > > >
> > > > qemu-system-x86_64 \
> > > >    [...] \
> > > >         -tpmdev 
> > > > cuse-tpm,id=tpm0,cancel-path=/dev/null,path=/dev/vtpm-test
> > \
> > > >         -device tpm-tis,id=tpm0,tpmdev=tpm0 \
> > > >    [...]
> > > >
> > > >
> > > > Signed-off-by: Stefan Berger <address@hidden>
> > > > Cc: Eric Blake <address@hidden>
> > >
> > > Before we add a dependency on this interface,
> > > I'd rather see this interface supported in kernel
> > > and not just in CUSE.
> > 
> > For using the single hardware TPM, we have the passthrough type. It's usage 
> > is
> > limited.
> > 
> > CUSE extends the TPM character device interface with ioctl's. Behind the
> > character device we can implement a TPM 1.2 and a TPM 2. Both TPM
> > implementations require large amounts of code, which I don't think should go
> > into the Linux kernel itself. So I don't know who would implement this
> > interface inside the Linux kernel.
> > 
> >   Stefan
> > 
> 
> BTW I'm not talking about the code - I'm talking about the interfaces here.
> 
> One way would be to add support for these interface support in the kernel.
> 
> Maybe others can be replaced with QMP events so management
> can take the necessary action.
> 
> As long as this is not the case, I suspect this code will have to stay
> out of tree :( We can't depend on interfaces provided solely by cuse
> devices on github.

The kernel already has a userspace device interface for TPMs doesn't
it - it is what our existing 'tpm-passthrough' backend in QEMU surely
uses.

If swtpm is going to the trouble of providing device node emulation
with CUSE, I would have thought it could emulate the same interface
as the existing kernel TPM device nodes, thus removing the need for
any extra driver in QEMU ?  Otherwise it doesn't seem like there's
much point in using CUSE, as opposed to a general userspace RPC
protocol that doesn't need kernel support at all.

Regards,
Daniel
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