Re: [Qemu-devel] [RESEND PATCH v3 0/5] Add UUID support.

[Jamie Lokier <jamie@shareable.org>]

Gleb Natapov wrote:
> > I don't like this vmport "backdoor" and I would like to disable it by
> > default, so I consider it is not the solution to pass the UUID
> > information to the BIOS.
>
> Do you have other interface in mind for host/guest communication?
> VMware uses the I/O port backdoor interface. Microsoft Virtual PC
> uses "Invalid Opcode" mechanism as a backdoor.
>
> We need this interface for other things too. For instance there are OEM
> Windows versions that require specific vendor's ACPI table to be present
> for installation. I have a patch that pass additional ACPI tables to BIOS
> using backdoor interface since it's not practical to compile different
> BIOSes for different Windows OEM versions.
>
> > BTW, what is the use of this UUID ?
>
> It is used for system management to tell two identical computers apart.
> Something like MAC but more stable. In Windows WMI can be used to obtain
> UUID and we have vendors that use VB scripts to configure machines
> differently based on their UUIDs.

Also, if you're porting an exiting Windows VM image from some other VM
system, the existing image will expect certain things which are UUIDs
(BIOS asset tags etc.) to have particular values.  If they are not
present, or the values are different, then the VM image may refuse to
run and ask you to "activate" it with a phone call to Microsoft.

I don't like this, but being able to port existing images makes a
difference when considering whether to use a Linux VM host or a
Windows VM host, to host Windows guests.

(Admittedly some other things also need to be emulated to seamlessly
take a Windows image from MS Virtual PC into QEMU.)

...

On the other hand, VM backdoors are a way in which OSes can refuse to
run if they detect they are in particular VMs, or behave differently.
This could be ugly: imagine an OS which runs fine in VMware and MS
Virtual PC, but refuses to run in QEMU/KVM _only_ because it detects
what it's running in, and not for technical reasons.

So it's important than VM backdoors are emulated as carefully as other
hardware, and don't expose too much real information about the host,
just the minimum required and under user control.

-- Jamie