Re: [PATCH] tpm: Add driver for TPM over virtio

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Mon, Feb 25, 2019 at 12:20:43PM -0800, James Bottomley wrote:
> On Mon, 2019-02-25 at 11:17 -0800, Matthew Garrett wrote:
> > On Mon, Feb 25, 2019 at 7:36 AM James Bottomley
> > <James.Bottomley@hansenpartnership.com> wrote:
> > > > The virtio driver performs discovery via virtio, which crosvm
> > > > implements already for all of its supported devices. This
> > > > substantially reduces the amount of TPM-specific code compared to
> > > > your suggestions, and lowers the barrier to entry for
> > > > implementing TPM support in other hypervisors which I hope we
> > > > agree is beneficial.
> > > 
> > > Well, that's somewhat misleading:  The reason we already have two
> > > hypervisor specific drivers already is because every hypervisor has
> > > a different  virtual discovery mechanism. You didn't find the other
> > > two hypervisor drivers remotely useful, so why would another
> > > hypervisor find yours useful?
> >  
> > The existing hypervisor drivers expose hypervisor-specific details.
> > This proposed driver provides an abstract interface that is usable by
> > other hypervisors. It allows building a VM that exposes TPM
> > functionality without requiring additional hardware emulation,
> > reducing the hypervisor attack surface.
> 
> Well, that depends whether you think a virtio bus is an abstract
> concept or a hypervisor specific detail.  There are currently four
> major hypervisors: xen, kvm, hyper-v and ESX.  Of those, only one
> implements virtio: kvm.  I agree virtio is a standard and certainly a
> slew of minor hypervisors implement it because they need paravirt
> support on Linux so they piggyback off kvm, but I don't see any of the
> other major hypervisors jumping on the bandwagon.
> 
> I certainly agree our lives would be easier if all the major hypervisor
> vendors would just agree a single paravirt driver standard.

I think that a Windows hypervisor (Hyper-V) and a closed hypervisor
(VMWare) are out of context for this discussion. I think it is a good
thing that there exist a fully open alternative to closed solutions such
as VMBus. It is not only good for Linux but also for other open source
operating systems (*BSD, Fuchsia etc.). I won't disregard virtio-TPM
based on that.

The main interest lies in these:

- QEMU
- KVM
- Xen

> >  Using the more generic virtio
> > infrastructure reduces the need for that, since any hypervisor should
> > be able to implement the backend (eg, in this case it'd be very easy
> > to add support for this driver to qemu,
> 
> I certainly agree there ... is there a plan for this?

I don't *necessarily* require QEMU to support this in the implementation
level in order to accept the change. What I do require is a buy-in from
the QEMU and Xen community that this is the right path.

/Jarkko