Re: [PATCH RESEND v6 00/36] Initial support for multi-process qemu

[Michael S. Tsirkin]

On Tue, Apr 28, 2020 at 06:29:20PM +0100, Stefan Hajnoczi wrote:
> On Wed, Apr 22, 2020 at 09:13:35PM -0700, address@hidden wrote:
> > We will post separate patchsets for the following improvements for
> > the experimental Qemu multi-process:
> >  - Live migration;
> >  - Asynchronous communication channel;
> >  - Libvirt support;
> > 
> > We welcome all your ideas, concerns, and questions for this patchset.
> 
> This patch series does two things:
> 1. It introduces the remote device infrastructure.
> 2. It creates the remote device program and the associated build changes
>    (makefiles, stubs, etc).
> 
> There are many patches and it's likely that a bunch more revisions will
> be necessary before this can be merged.
> 
> I want to share an idea to reduce the scope and get patches merged more
> quickly.  It looks like the series can be reduced to 21 patches using
> this approach.
> 
> I suggest dropping the remote device program from this patch series (and
> maybe never bringing it back).  Instead, use the softmmu target for the
> remote device.
> 
> Why?  Because the remote device program is just a QEMU that uses the
> remote machine type and has no vCPUs:
> 
>   $ qemu-system-x86_64 -chardev id=char0,... \
>                        -M remote,chardev=char0 \
>                      -device lsi53c810 \
>                      -drive if=none,id=drive0,file=vm.img,format=raw \
>                      -device scsi-hd,drive=drive0
> 
> This will use the remote machine type, interrupt controller, and PCI bus
> that you have created.
> 
> The remote machine type should default to no vCPUs and no memory
> creation (the memory comes via the mpqemu link communications channel).
> 
> At this point qemu-system-x86_64 contains a lot of code that you don't
> want in the final remote device program.  Let's ignore that for a
> second.
> 
> Now you can submit a 21-patch series containing just the remote device
> infrastructure.  This will be easier to merge.
> 
> Returning to code size, the next step is to reduce the binary.  QEMU has
> a Kconfig-style system for optional features and dependencies.  It's a
> better approach than creating a separate make target because it
> eliminates the duplication and mess in the makefiles.
> 
> For example, you can disable TCG and KVM so that your binary has no
> ability to execute guest code.  Currently ./configure disallows this but
> I've tried it and it works.
> 
> You can add a new default-configs/ file that disables CONFIG_ISAPC,
> CONFIG_I440FX, etc.  When you compile QEMU most of hw/ will not be built
> anymore.  At this point you have a smaller binary that is still a
> softmmu target so the makefiles are shared with the regular
> qemu-system-x86_64.
> 
> There will be some code for which there is no Kconfig option yet.
> Further improvements can be made by adding Kconfig options for any code
> that you wish to eliminate.  Instead of writing makefile changes like
> you did in this patch series you would be adding Kconfig options.  The
> nice thing is that this work isn't specific to the remote device program
> - anyone can use the new Kconfig options to reduce the size of their
> QEMU.  So not only is it less messy than duplicating the makefiles,
> but it also benefits everyone.
> 
> The downside to doing this is that it will take a while to eliminate all
> code that you don't want via Kconfig.  However, your initial patch
> series can be merged sooner and I think this direction is also cleaner.
> 
> I hope I've explained the idea properly :).  We can continue reviewing
> the current series if you prefer, but I think it would be quicker to
> drop the remote device program.
> 
> Stefan

Building QEMU twices just to get the remote is however not very
attractive. So how about making remote a special target?
Either remote-softmmu/ or if impossible x86_64-remote-softmmu/

-- 
MST