Re: [PATCH experiment 00/16] C++20 coroutine backend

[Dr. David Alan Gilbert]

* Paolo Bonzini (pbonzini@redhat.com) wrote:
> On 3/15/22 16:55, Daniel P. Berrangé wrote:
> > Expecting maintainers to enforce a subset during code review feels
> > like it would be a tedious burden, that will inevitably let stuff
> > through because humans are fallible, especially when presented
> > with uninspiring, tedious, repetitive tasks.
> > 
> > Restricting ourselves to a subset is only viable if we have
> > an automated tool that can reliably enforce that subset. I'm not
> > sure that any such tool exists, and not convinced our time is
> > best served by trying to write & maintainer one either.
> 
> We don't need to have a policy on which features are used.  We need to have
> goals for what to use C++ for.  I won't go into further details here,
> because I had already posted "When and how to use C++"[1] about an hour
> before your reply.
> 
> > IOW, I fear one we allow C++ in any level, it won't be practical
> > to constrain it as much we desire. I fear us turning QEMU into
> > even more of a monster like other big C++ apps I see which take
> > all hours to compile while using all available RAM in Fedora RPM
> > build hosts.
> 
> Sorry but this is FUD.  There's plenty of C++ apps and libraries that do not
> "take hours to compile while using all available RAM".  You're probably
> thinking of the Chromium/Firefox/Libreoffice triplet but those are an order
> of magnitude larger than QEMU.  And in fact, QEMU is *already* a monster
> that takes longer to compile than most other packages, no matter the
> language they're written in.
> 
> Most of KDE and everything that uses Qt is written in C++, and so is
> Inkscape in GTK+ land.  LLVM and Clang are written in C++.  Hotspot and V8
> are written in C++.  Kodi, MAME and DolphinEmu are written in C++. GCC and
> GDB have migrated to C++ and their compile times have not exploded.

While I think it does take longer to compile, the bigger problem for
the CI setup is the amount of RAM-per-compile-process; it's not so much
the fact that those applications are huge that's the problem, it's that
a make -j ($threads) can run out of RAM.

Dave

> > My other question is whether adoption of C++ would complicate any
> > desire to make more use of Rust in QEMU ? I know Rust came out of
> > work by the Mozilla Firefox crew, and Firefox was C++, but I don't
> > have any idea how they integrated use of Rust with Firefox, so
> > whether there are any gotcha's for us or not ?
> 
> Any Rust integration would go through C APIs.  Using Rust in the block layer
> would certainly be much harder, though perhaps not impossible, if the block
> layer uses C++ coroutines.  Rust supports something similar, but
> two-direction interoperability would be hard.
> 
> For everything else, not much.  Even if using C++, the fact that QEMU's APIs
> are primarily C would not change.  Changing "timer_mod_ns(timer, ns)" to
> "timer.modify_ns(ns)" is not on the table.
> 
> But really, first of all the question should be who is doing work on
> integrating Rust with QEMU.  I typically hear about this topic exactly once
> a year at KVM Forum, and then nothing.  We have seen Marc-André's QAPI
> integration experiment, but it's not clear to me what the path would be from
> there to wider use in QEMU.
> 
> In particular, after ~3 years of talking about it, it is not even clear:
> 
> - what subsystems would benefit the most from the adoption of Rust, and
> whether that would be feasible without a rewrite which will simply never
> happen
> 
> - what the plans would be for coexistence of Rust and C code within a
> subsystem
> 
> - whether maintainers would be on board with adopting a completely different
> language, and who in the community has enough Rust experience to shepherd us
> through the learning experience
> 
> The first two questions have answers in the other message if s/Rust/C++/,
> and as to the last I think we're already further in the discussion.
> 
> Thanks,
> 
> Paolo
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK