Re: Device model architecture (Was Re: Re: Are linkerscripts needed?)

[Arun Sharma <arun.sharma@intel.com>]

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