Re: [Qemu-devel] [PATCH v3 0/4] Introduce the microvm machine type

[Paolo Bonzini]

On 25/07/19 12:03, Michael S. Tsirkin wrote:
>> +#ifdef CONFIG_PCI
>> +    x86_platform.pci_scan_bus = kvm_pci_scan_bus;
>> +#endif
>> +
>>      if (!kvm_para_available())
>>              return;
>>  
> Shouldn't this happen after kvm_para_available?

Actually kvm_para_available() is not needed anymore, since this only
runs after kvm_detect() has returned true.

> In fact, let's add a CPU ID flag for this, so it's
> easy to tell guest whether to scan extra buses.
> What do you say?

I think it would make it much harder to deploy this, since it relies on
having new userspace and new machine types.  This patch is basically a
reflection of the status quo, which is that there are generally no
"hidden" buses on commonly-used KVM userspaces, and even in the weird
configurations that have them there is always something at devfn=0.

(On real hardware, the only such hidden bus is e.g. 0x7f/0xff, which
have a bunch of QPI and MCH-related devices.  This is not something
you'd have in a virtual machine).

Paolo