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xen-devel
[Xen-devel] Full virtualization and I/O
Hi
Full virtualization is about providing multiple virtual ISA level
environments and mapping them to a single physical one. One particular
aspect of this mapping are I/O instructions (explicit or mmapped I/O). In
general, there are two strategies to partition the devices, either in time
or in space. Partitioning a device in space means that the device (or a
part of it) is exclusively available to a single VM. Partitioning a device
in time (or time multiplexing) means that it can be used by multiple VMs
but only one VM may use it at any point in time.
I am trying to understand how I/O virtualization on the ISA level works if
a device is shared between multiple VM instances. On a very high level, it
should be as follows. First of all, the VMM has to intercept the VM's I/O
commands (I/O instructions or load/store to dedicated memory addresses -
let's ignore interrupts for the moment). This could be done by traps or by
replacing the resp. instructions by VMM calls to I/O primitives. The VMM
keeps multiple device model instances (one for each VM using the device)
in memory. The models somehow reflect the low level I/O API of the device.
Depending on which I/O command is issued by the VM, either the memory
model is changed or a number of I/O instructions are executed to make the
physical device state reflect the one represented in the memory model.
This approach brings up a number of questions. It would be great if some of
the virtualization experts here could shed some light on them (even though
they are not immediately related to Xen, I know):
- How do these device memory models look like? Is there a common
(automata) theory behind or are they done ad hoc?
- What kind of strategies/algorithms are used in the merge phase, i.e. the
phase where the virtual memory model and the physical one are
synchronized? What kind of problems can occur in this phase?
- Are specific usage patterns used in real world implementations (e.g.
VMWare) to simplify the virtualization (model or merge phase)?
- Do you have any interesting pointers to literature dealing with full I/O
virtualization? In particular, how does VMWare's full virtualization
works with respect to I/O?
- Is every device time partitionable? If not, which requirements does it
have to meet to be time partitionable?
-> I don't think every device is. What about a device which supports
different modes of operation. If two VMs drive the virtual device in
different modes, it may not be possible to constantly switch between
them. Ok, this is pretty artificial.
Thanks a lot for your help!
Best wishes
Thomas
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