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

[Arun Sharma <arun.sharma@xxxxxxxxx>]

Ian Pratt wrote:
> > I guess the main logic behind your argument is that there is 
> > no need to fully virtualize the device models, so no need to 
> > run them within a non-root VMCS.
>
> We can't run them in the same non-root VMCS as the guest since we need
> some virtual address space. [...]
You can, with a world switch using a different CR3.

> > The only comment I have is: for the case where the device 
> > models require the services of a backend driver, you might be 
> > paying more than what we currently pay (one domain switch) 
> > i.e. vmx domain -> mini os -> backend. But it should be 
> > faster for everything else.
>
> The vmx_domain to mini-OS switch happens as part of the vmexit.
It's actually the vmexit + cost of upcall to the mini-os + cost of 
domain switch to the backend (say dom0).
Right now, it's vmexit + cost of domain switch to dom0 + cost of dom0 
kernel to user mode device model.
So the difference is

cost of dom0 kernel to user - cost of frontend to backend switch

I suspect that the delta is small and not much to worry about.

> Devices such as the APIC/IOAPIC/PIT etc can all be emulated without
> calling out of the device model, and should work with just the same
> performance as having them in Xen as we do today.
True, it's definitely a win for devices that don't require communication 
with the backend.
Also, some customers might want to use the split I/O model, where the 
VMX domain directly wants to use a raw partition on real disk. I'd 
rather have drivers in the OS hosting the device-models instead of going 
through the backend.
        -Arun

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