Re: Device model architecture (Was Re: [Xen-devel] Re: Are linker scripts needed?)

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 24 May 2005, at 19:28, Arun Sharma wrote:

> Ian Pratt wrote:
> > I'd be inclined to move to a model where we execute the device 
> > emulation
> > in the root (monitor) VMCS, using the same protection mechanism we use
> > for para-virtualized guests e.g. segmentation for x86, paging for
> > x86_64. The device emulation should should work like a normal 
> > front-end
> > driver, connecting via a device channel to a normal backend.
> It sounded like you were proposing linking the device models against 
> Xen.
No, Ian is suggesting running them in 'paravirtualised mode': probably 
in ring 3 of the root VMCS. So there will be three contexts that a VMCS 
domain runs:
 1. Full-virt context (guest VMCS, rings 0-3)
 2. Hypervisor context (root VMS, ring 0)
 3. Paravirt driver context (root VMCS, rings 1-3)

> - For every VMX domain created, create a new helper domain
> - The helper domain shares it's page list with the VMX domain
> - xen is protected from the helper domain using paging/segmentation
> - helper domain runs minios
> - Use the existing mechanisms (backend drivers) to get storage/network 
> services from dom0
Yes.

> - why is this better than running the device models inside the VMX 
> domain? Do you expect switching to the helper domain to be faster than 
> a  vmx world switch?
Depends on whether you can make the CPU do a direct switch, or if you 
need to 'bounce' through root VMCS (taking an extra cr3 switch).
> - what's the advantage of running minios vs xenolinux in the helper 
> domain?
Full xenolinux is totally unnecessary. We just want enough support 
services to run the device emulator and front-end driver stub. We're 
best building those from scratch -- it'll look nothing like an 
operating system.
> I think we all agree that:
>
> - It'd be good to make the device models "embeddable" so that it could 
> be moved closer to the domain it's servicing. This is where the bulk 
> of the work is, regardless of which model we end up choosing.
> - Make sure that there is a unified way to manage the resources given 
> to the VMX domain (including the device models)
Yes, agreed.

 -- Keir


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