Re: [PATCH 05/22] hw/arm: Select VIRTIO_NET for virt machine

[Paolo Bonzini]

On 5/9/23 13:37, Alex Bennée wrote:
> > Then, an accurate description that uses "functional" in that sense
> > could be as follows:
> >
> > The Kconfig system will include any devices and subsystems that are
> > mandatory for a given machine type, and will flag any configuration     
> > conflicts. However, the person doing the configuration still needs to
> > know which devices are needed (on top of the mandatory ones) to obtain
> > a functional guest, and Kconfig will not provide any hints in this
> > respect.
>
> So I thought that was the model I was following in adding devices but it
> seems I don't understand the no-defaults behaviour. What is the
> difference between a device that is added in the machine.c that makes it
> required or expendable with -nodefaults?

First of all let's look at softmmu/vl.c, which is what creates the 
backends of the default devices.  These backends are basically automatic 
command line options of the "old" kind that directs both the creation of 
a backend and how it's associated to a device.

For example, if you have neither a -serial option nor a "-device 
isa-serial" option, vl.c's qemu_create_default_devices() will create the 
machine as if you specified one of "-serial mon:stdio", "-serial stdio" 
or "-serial vc:80Cx24C"; if you have none of -netdev/nic/-net, it will 
literally add two "-net" command line options corresponding to "-net nic 
-net user"; and so on for many other devices unless the board opts out 
(CD-ROM, floppy, SD card, parallel).   There's also a default monitor 
which is a bit out of topic here.

In any case, the effect of these "old" command line options is to 
populate various arrays, for example serial_hds[] for serial ports and 
nd_table[] for NIC backends.

There are then two ways that a board can process these arrays.  It can 
either _always_ create the device and possibly leave it not connected to 
any backend, or it can create the device only if the backend exists. 
Looking at qemu-system-aarch64's "-M virt", pl011 is always created with 
create_uart(), i.e. even with "-nodefaults"; NICs instead are only 
created if the backend exists (search for "nd_table" in hw/arm/virt.c).

In the former case, the device will have to be select-ed in Kconfig.  In 
the latter case, instead, the device will have an "imply" directive, and 
will be left out by --without-default-devices.

Paolo