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

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "ehabkost@redhat.com" <ehabkost@redhat.com>,
	Sergio Lopez <slp@redhat.com>,
	"maran.wilson@oracle.com" <maran.wilson@oracle.com>,
	"Montes, Julio" <julio.montes@intel.com>,
	Stefan Hajnoczi <stefanha@gmail.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"kraxel@redhat.com" <kraxel@redhat.com>,
	"sgarzare@redhat.com" <sgarzare@redhat.com>,
	"rth@twiddle.net" <rth@twiddle.net>
Subject: Re: [Qemu-devel] [PATCH v3 0/4] Introduce the microvm machine type
Date: Thu, 25 Jul 2019 12:55:13 +0200	[thread overview]
Message-ID: <390698b1-03a2-2488-bd55-e3a90c8d0638@redhat.com> (raw)
In-Reply-To: <20190725060105-mutt-send-email-mst@kernel.org>

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