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 06:06:02PM +0000, Dr. David Alan Gilbert wrote:
> > * 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
> 
> Separate process we start and stop automatically I don't mind. A
> separate tree with a distinct coding style where no one will ever even
> look at it? Not so much.

That code is used elsewhere anyway, so asking them to change the coding style
isn't very nice.
Even if they change the coding style it doesn't mean you're suddenly going to
understand how a TPM works in detail and be able to review it.

Anyway, having it in a separate process locked down by SELinux means that even
if it does go horribly wrong it won't break qemu.

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.
> > > 
> > > > -- 
> > > > |: http://berrange.com      -o-    
> > > > http://www.flickr.com/photos/dberrange/ :|
> > > > |: http://libvirt.org              -o-             
> > > > http://virt-manager.org :|
> > > > |: http://entangle-photo.org       -o-    
> > > > http://search.cpan.org/~danberr/ :|
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK