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

[Dr. David Alan Gilbert]

* Michael S. Tsirkin (address@hidden) wrote:
> On Wed, Mar 01, 2017 at 05:38:23PM +0000, Daniel P. Berrange wrote:
> > On Wed, Mar 01, 2017 at 12:25:46PM -0500, Stefan Berger wrote:
> > > On 03/01/2017 12:16 PM, 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.
> > > 
> > > You mean put the code implementing TPM 1.2 and/or TPM 2 into the QEMU 
> > > tree?
> > > These are multiple thousands of lines of code each and we'll break them
> > > apart into logical chunks and review them?
> > 
> > No, lets not make that mistake again - we only just got rid of the
> > libcacard smartcard library code from QEMU git. 
> > 
> > Regards,
> > Daniel
> 
> I don't mean that as an external library. As an integral part of QEMU
> adhering to our coding style etc - why not?
> 
> I don't know what are the other options.  How is depending on an ABI
> with a utility with no other users and not packaged by most distros
> good? You are calling out to a CUSE device but who's reviewing that
> code?
> 
> vl.c weights in a 4500 lines of code. several thousand lines is
> not small but not unmanageable.


That's 4500 lines of fairly generic code; not like the TPM where the number
of people who really understand the details of it is pretty slim.

It's better on most counts to have it as a separate process.

Dave

> Anyway, it all boils down to lack of reviewers. I know I am not merging
> the current implementation because I could not figure out what do qemu
> bits do without looking at the implementation. I don't want to jump
> between so many trees and coding styles. bios/qemu/linux/dpdk are
> painful enough to manage. If some other maintainer volunteers, or if
> Peter wants to merge it directly from Stefan, I won't object.
> 
> > -- 
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK