Virtual Machines and Devices/Drivers

[Jordus]

I see a lot of questions on drivers and devices for Virtual machines. (Hyper-V, VMware, "XP mode", Virtual PC, etc)

Its important to understand that the devices the virtual machines see are not the actual hardware in the machine.

That is to say...the virtual machine might know that there is a network card available to use, but it doesnt know that the physical network card is a Broadcom 3c905 (for example). The virtual machine thinks that it is a pre-defined device that is written specifically for use in the virtual machine.

Same goes with other hardware, your Nvidia 9800 GTX would show up in the VM as a totally different piece of hardware. And this virtual piece of hardware does not have the features or power of the 9800 GTX nor does it require its device drivers.

Virtual machines use 2 types of devices. Emulated and Synthetic.

Emulated - Drivers/Devices that require a lot of physical box overhead to process for the virtual machine. Basically there is a lot of translation that the hypervisor (virtual pc, hyper-v, etc) must perform for the virtual device to utilize the physical device. These are the types of devices/drivers you will see used in Virtual PC and MS Virtual Server 2005 and some others.

Synthetic - Virtual Devices/Drivers that are highly optimized to perform very seamlessly with the underlying physical device. Hypervisor intervention/translation is kept minimal and therefore performance is greatly improved. Virtual machines using synthetic drivers are often said to be "enlightened". These are generally seen in bare metal hypervisors like Hyper-V and ESXi.


The important thing to take away from this post is that the actual physical devices in your computer are NOT what the virtual machine sees.

The network traffic from your virtual machine is passing through the physical network card, but the virtual machine will "see" a totally different device than what is actually in the computer. This goes for any device.

Hopefully this will clear up some confusion.


EDIT: Some additional reading if anyone would like

http://searchservervirtualization.techtarget.com/tip/0,289483,sid94_gci1312278,00.html
http://windowsitpro.com/article/articleid/99279/hyper-v-synthetic-devices.html

 

[swarfega]

Good explanation, this should be stickied.

 

[carioca]

<SNIP>
The important thing to take away from this post is that the actual physical devices in your computer are NOT what the virtual machine sees. <SNIP>

I think I am getting your point - but what about the actual non-existant devices in my computer that XP mode sees? E.g. in XP mode my drives include a floppy disk drive A: which physically doesn't exist in my machine... why do I score a virtual floppy? What can I do with that?

Still confused...

Cheers,

LMH

 

[aem]

I'm of the opinion that a physical devices is unique interms of how the physical system sees them. For VM to be able to integrate with these devices they have to use emulation technology. You won't find device Ids (PCIs) to be idential in real machines and in VMs. At least i have not come across any.

"The important thing to take away from this post is that the actual physical devices in your computer are NOT what the virtual machine sees."

I'm not too certain this is entirely true. To be able to emulate devices, VMs needs to know for example what a DVD-ROM is and what a Floppy is. VM however has the ability to emulate/create pseudo devices which is why they are called Virtual Machines.

As for why there is a Floppy drive in XPM, apart from just been a Virtual thingie, there are apps that needs to be run from Floppy, you can mount a Floppy drive as you do a HDD. Crazy as it sound but that's the way it is.

 

[Jordus]

<SNIP>
The important thing to take away from this post is that the actual physical devices in your computer are NOT what the virtual machine sees. <SNIP>

I think I am getting your point - but what about the actual non-existant devices in my computer that XP mode sees? E.g. in XP mode my drives include a floppy disk drive A: which physically doesn't exist in my machine... why do I score a virtual floppy? What can I do with that?

Still confused...

Cheers,

LMH

Its the same as any other virtual device. Actually, if you DIDNT have a floppy in your physical machine you can still add one in your VM. You can also add DVD/CD-roms even if you dont have a physical one, and just mount ISOs instead of physical discs.