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

[Daniel P. Berrange]

On Wed, Mar 01, 2017 at 07:16:01PM +0200, Michael S. Tsirkin wrote:
> On Wed, Mar 01, 2017 at 12:12:34PM -0500, Stefan Berger wrote:
> > On 03/01/2017 12:02 PM, Michael S. Tsirkin wrote:
> > > On Wed, Mar 01, 2017 at 04:31:04PM +0000, Daniel P. Berrange wrote:
> > > > On Wed, Mar 01, 2017 at 06:22:45PM +0200, Michael S. Tsirkin wrote:
> > > > > On Wed, Mar 01, 2017 at 09:50:38AM -0500, Stefan Berger wrote:
> > > > > > I had already proposed a linked-in version before I went to the 
> > > > > > out-of-process
> > > > > > design. Anthony's concerns back then were related to the code not 
> > > > > > being trusted
> > > > > > and a segfault in the code could bring down all of QEMU. That we 
> > > > > > have test
> > > > > > suites running over it didn't work as an argument. Some of the test 
> > > > > > suite are
> > > > > > private, though.
> > > > > Given how bad the alternative is maybe we should go back to that one.
> > > > > Same argument can be made for any device and we aren't making
> > > > > them out of process right now.
> > > > > 
> > > > > IIMO it's less the in-process question (modularization
> > > > > of QEMU has been on the agenda since years and I don't
> > > > > think anyone is against it) it's more a code control/community 
> > > > > question.
> > > > I rather disagree. Modularization of QEMU has seen few results
> > > > because it is generally a hard problem to solve when you have a
> > > > complex pre-existing codebase.  I don't think code control has
> > > > been a factor in this - as long as QEMU can clearly define its
> > > > ABI/API between core & the modular pieces, it doesn't matter
> > > > who owns the module. We've seen this with vhost-user which is
> > > > essentially outsourcing network device backend impls to a 3rd
> > > > party project.
> > > And it was done precisely for community reasons.  dpdk/VPP community is
> > > quite large and fell funded but they just can't all grok QEMU.  They
> > > work for hardware vendors and do baremetal things.  With the split we
> > > can focus on virtualization and they can focus on moving packets around.
> > > 
> > > 
> > > > QEMU's defined the vhost-user ABI/API and delegated
> > > > impl to something else.
> > > The vhost ABI isn't easy to maintain at all though. So I would not
> > > commit to that lightly without a good reason.
> > > 
> > > It will be way more painful if the ABI is dictated by a 3rd party
> > > library.
> > 
> > Who should define it?
> 
> No one. Put it in same source tree with QEMU and forget ABI stability
> issues.

That doesn't work very well in practice as you have to make sure the
vTPM process that is running, provides exactly the same ABI as the QEMU
process that's connecting to it. You could have a single vTPM process
on the host serving many QEMU processes, each of which could be a
different QEMU version, due to upgraded RPMs/Debs.

Regards,
Daniel
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