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

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


- 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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