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

[Daniel P. Berrange]

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. QEMU's defined the vhost-user ABI/API and delegated
impl to something else.

With the vTPM stuff here, we've not got a pre-existing feature
we need to deal with, so the biggest blocker wrt modularization does
not exist. Given that I think having the vTPM impl modularized is
highly desirable, as long as we can define a sane ABI/API between
QEMU and the external piece.  So I think anthony's point about not
putting a vTPM impl in-process is still valid, and since Stefan's
already done much of the work to achieve a modular design we should
not go back to an in-process design now.

> It doesn't look like userspace swtpm bits have a large community of
> developers around it, and the only user appears to be QEMU, so depending
> on that externally does not make sense, we should just have them
> in-tree. This way we don't need to worry about versioning etc.

Regards,
Daniel
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