Re: [PATCH 1/1] hw/s390x: modularize virtio-gpu-ccw

[Halil Pasic <pasic@linux.ibm.com>]

On Thu, 18 Feb 2021 10:23:16 +0100
Thomas Huth <thuth@redhat.com> wrote:

> > Since the virtio-gpu-ccw device depends on the hw-display-virtio-gpu
> > module, which provides the type virtio-gpu-device, packaging the
> > hw-display-virtio-gpu module as a separate package that may or may not
> > be installed along with the qemu package leads to problems. Namely if
> > the hw-display-virtio-gpu is absent, qemu continues to advertise
> > virtio-gpu-ccw, but it aborts not only when one attempts using
> > virtio-gpu-ccw, but also when libvirtd's capability probing tries
> > to instantiate the type to introspect it.
> > 
> > Let us thus introduce a module named hw-s390x-virtio-gpu-ccw that
> > is going to provide the virtio-gpu-ccw device. The hw-s390x prefix
> > was chosen because it is not a portable device.
> > 
> > With virtio-gpu-ccw built as a module, the correct way to package a
> > modularized qemu is to require that hw-display-virtio-gpu must be
> > installed whenever the module hw-s390x-virtio-gpu-ccw.
> > 
> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> > ---
> >   hw/s390x/meson.build | 17 ++++++++++++++++-
> >   util/module.c        |  1 +
> >   2 files changed, 17 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/s390x/meson.build b/hw/s390x/meson.build
> > index 2a7818d94b..153b1309fb 100644
> > --- a/hw/s390x/meson.build
> > +++ b/hw/s390x/meson.build
> > @@ -34,7 +34,6 @@ virtio_ss.add(files('virtio-ccw.c'))
> >   virtio_ss.add(when: 'CONFIG_VIRTIO_BALLOON', if_true: files('virtio-ccw-balloon.c'))
> >   virtio_ss.add(when: 'CONFIG_VIRTIO_BLK', if_true: files('virtio-ccw-blk.c'))
> >   virtio_ss.add(when: 'CONFIG_VIRTIO_CRYPTO', if_true: files('virtio-ccw-crypto.c'))
> > -virtio_ss.add(when: 'CONFIG_VIRTIO_GPU', if_true: files('virtio-ccw-gpu.c'))
> >   virtio_ss.add(when: 'CONFIG_VIRTIO_INPUT', if_true: files('virtio-ccw-input.c'))
> >   virtio_ss.add(when: 'CONFIG_VIRTIO_NET', if_true: files('virtio-ccw-net.c'))
> >   virtio_ss.add(when: 'CONFIG_VIRTIO_RNG', if_true: files('virtio-ccw-rng.c'))
> > @@ -46,3 +45,19 @@ virtio_ss.add(when: 'CONFIG_VHOST_USER_FS', if_true: files('vhost-user-fs-ccw.c'
> >   s390x_ss.add_all(when: 'CONFIG_VIRTIO_CCW', if_true: virtio_ss)
> >   
> >   hw_arch += {'s390x': s390x_ss}
> > +
> > +if target.startswith('s390x')
> > +  hw_s390x_modules = {}
> > +
> > +  hw_s390x_modules_c_args = ['-DNEED_CPU_H',
> > +	      '-DCONFIG_TARGET="@0@-config-target.h"'.format(target)]
> > +  hw_s390x_modules_inc = [include_directories('../../target' / config_target['TARGET_BASE_ARCH'])]
> > +  hw_s390x_modules_dependencies = declare_dependency(
> > +	       include_directories: hw_s390x_modules_inc, compile_args: hw_s390x_modules_c_args)  
> 
> Basically the patch looks fine to me, but I wonder why all that above lines 
> (related to hw_s390x_modules_dependencies) are requred at all? The other 
> display modules in hw/display/meson.build also do not need to re-define 
> c_args for example?

The explanation is simple. Unlike most devices, the ccw devices aren't
portable. In particular both css.c and css.h includes "cpu.h", and
virtio-ccw-gpu.c includes "qemu/osdep.h". Furthermore osdep.h contains:
#ifdef NEED_CPU_H
#include CONFIG_TARGET
#else
#include "exec/poison.h"
#endif
so if we don't have NEED_CPU_H, among others CONFIG_KVM is poisoned, and
CONFIG_KVM is used in "css.h". Frankly, I can't tell under what circumstances
does css need "cpu.h". I managed to build the s390x-softmmu target
without it, but decided to put it back. Regarding "osdep.h", I just
assumed includes are done the way they are done for a good reason. Maybe
the includes can be changed in a way that the things you ask about become
unnecessary, but with the code as is they are necessary. Try to drop them
and check out what happens.

Regards,
Halil