Re: [Xen-users] On Wndows, Video and Sound

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Hard disks can be simulated in several different ways. The most simple
> way to do it would be using a FAT32/NTFS formatted file in QEMU. QEMU
> can use a file as a disk. At this level of operation, QEMU just takes
> disk read/write/etc operations and transfer them to a Linux file-system
> call. This means that you can not really use /var/Windows/... under
> Linux, but you have a file in Linux that looks like a disk to the
> Windows OS - this will probably be a large file, several gigabytes if
> you want to do anything useful with it... ;-) The file can be on any
> file-system that supports read/write in Linux, so Ext1/2/3, Reiser, JFS,
> etc, etc.

The other option for exporting something like /var/Windows would be to emulate 
a "virtual FAT device" on top of a Linux filesystem.  This allows read only 
access to a host directory structure, by exporting it to the guest *as if* it 
were a FAT-formatted block device.  You could export 
a /var/ShareThisWithWindows read-only to the guest using this approach.

Under QEmu there's a patch to do this already, so it should be portable to the 
current device model code.  A good way to do it under Xen if we move away 
from the userspace device emulation might be to rewrite that code to use the 
blktap driver (then it'd work for paravirtualised domains too - if you really 
wanted to do that!).

> development that no one has done so far]. The hard disk interface on IDE
> may or may not "split" in the middle if you have a four-disk interface.
> Certainly, on a single controller (i.e. IDE0) you can not share the disk
> out, since it's actually using the SAME bits of wire to talking to the
> two disks. So for some cases, you haven't got a choice but let Dom0
> handle the hardware - unless you expect nVidia, ATI, chipset
> manufacturers and Microsoft to all of a sudden start producing "Xen
> aware" drivers.

I think hard drives are less of a pain, since they have reasonably high 
latencies even without virtualisation.  Graphics and network both have huge 
bandwidth and special state / latency requirements so they should definitely 
benefit from support in the peripherals themselves.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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