Re: [Qemu-devel] [PATCH] hw/s390x: Fix the function arguments in the pci stub file

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 1 Feb 2019 07:46:40 +0100
Thomas Huth <thuth@redhat.com> wrote:

> On 2019-02-01 07:14, Thomas Huth wrote:
> > On 2019-01-31 19:08, Paolo Bonzini wrote:  
> >> On 31/01/19 19:00, Thomas Huth wrote:  
> >>>>> (and the prototypes in the header) anymore, so if you try to compile s390x
> >>>>> without CONFIG_PCI, the build currently fails.
> >>>>>  
> >>>> Fixes: 468a93898a97 ("s390x/pci: pass the retaddr to all PCI instructions")
> >>>>  
> >>>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
> >>>>> ---
> >>>>>  hw/s390x/s390-pci-stub.c | 16 +++++++++-------
> >>>>>  1 file changed, 9 insertions(+), 7 deletions(-)  
> >>>> This file seems to be in danger of bitrot. Do you think it'll be easier
> >>>> to test rarely used configs like that after we switch to Kconfig?  
> >>> I hope so, yes. There will be a new --without-default-devices options
> >>> for "configure" (which matches "make allnoconfig" from the kernel) - if
> >>> we do it right in the Kconfig file for s390x, it should be possible to
> >>> catch this problem with that option.  
> >>
> >> Yes, it will be in .travis.yml too.
> >>
> >> Right now there is a "select PCI" in the hw/s390x/Kconfig file, but
> >> probably it's best to add a config S390_ZPCI with "default y if
> >> S390_CCW_VIRTIO" and "select PCI" in it.  Not a blocker, but I can
> >> integrate it if you send me a fixup patch.  
> > 
> > Yes, that's what I had in mind, too. I'll send a fixup patch...  
> 
> Actually, disabling the s390-pci code works from a compiling+linking
> point of view, but when you then try to start the machine, it fails:
> 
> $ s390x-softmmu/qemu-system-s390x -nographic
> qemu-system-s390x: Unknown device 's390-pcihost' for default sysbus
> Aborted (core dumped)
> 
> IIRC we originally wanted to make the "s390-pcihost" device really
> optional, but then decided not to do it since it would break migration.
> So the status of the "PCI disablement" got stuck somewhere inbetween,
> where we've created the switch for the Makefile and the stub file, but
> never really got it to a point where it could really really be disabled.

Well you can disable it, but it's not usable ;)

But yes, that annoying pcihost device needed for backwards compat got
into the way.

> 
> So I see two options now:
> 
> 1) Finally really make the device optional, at least for new machine
> types, so we can really disable CONFIG_PCI and get a working executable.
> 
> 2) Scratch the idea completely to make this optional, always link the
> s390-pci-bus.o and s390-pci-inst.o files unconditionally, and remove the
> s390-pci-stub.c file.
> 
> I assume options 2 is preferred, since we likely rather want to move
> into the PCI direction in the long run, instead of ignoring it...

I think both options are viable, but option 1 is of course more work.
The win there is that we could disable an entire subsystem.

I guess that the basic questions are: How important is it that
subsystems can be compiled out, and do we see a use case for a pci-less
s390 machine in the future? We really don't want to spend much time on
something of dubious use...