Re: [PATCH v15 15/60] i386/xen: add pc_machine_kvm_type to initialize XEN_EMULATE mode

[Paolo Bonzini]

On 3/10/23 09:28, David Woodhouse wrote:
> On Fri, 2023-03-10 at 11:15 +0800, Xiaoyao Li wrote:
> > On 3/1/2023 9:51 PM, David Woodhouse wrote:
> > > From: David Woodhouse <dwmw@amazon.co.uk>
> > >
> > > The xen_overlay device (and later similar devices for event channels and
> > > grant tables) need to be instantiated. Do this from a kvm_type method on
> > > the PC machine derivatives, since KVM is only way to support Xen emulation
> > > for now.
> >
> > Just curious, isn't there any more reasonable place to add machine
> > specific initialization?
> >
> > abusing the mc->kvm_type() looks bad to me.
>
> Hm, good question. Off the top of my head I have no better answer than
> "Paolo made me do it":
>
> https://lore.kernel.org/qemu-devel/8495140d-3301-7693-b804-0554166802da@redhat.com/
>
> But I have gained a bit more clue since December, and reading that
> message again I'll put a lot more focus on the fact that he said
> "during mc->kvm_type OR AFTERWARDS".

I don't think this is abusing mc->kvm_type; that is the point where 
startup code tells the machine "now you have your accelerator 
configuration, do what you want with that info".  In fact I find using 
xen_enabled() in mc->kvm_type to be less ugly than having a MachineClass 
entry just for KVM. :)

But, if you want to move it to pc_basic_device_init() that is certainly 
okay too.  It's not like people are going to add TCG/Xen support 
tomorrow but it is a tiny step in the direction of less 
accelerator-specific code for Xen emulation.

Paolo