Re: [Draft F] Xen on ARM vITS Handling

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2015-06-11 at 10:40 +0100, Ian Campbell wrote:
> ## Command Queue Virtualisation
> 
> The command translation/emulation in this design has been arranged to
> be as cheap as possible (e.g. in many cases the actions are NOPs),
> avoiding previous concerns about the length of time which an emulated
> write to a `CWRITER` register may block the vcpu.
> 
> The vits will simply track its reader and writer pointers. On write
> to `CWRITER` it will immediately and synchronously process all
> commands in the queue and update its state accordingly.
> 
> It might be possible to implement a rudimentary form of preemption by
> periodically (as determined by `hypercall_preempt_check()`) returning
> to the guest without incrementing PC but with updated internal
> `CREADR` state, meaning it will reexecute the write to `CWRITER` and
> we can pickup where we left off for another iteration. This at least
> lets us schedule other vcpus etc and prevents a monopoly.

In the presence of multiple VCPUs writing to GITS_CWRITER preemption
actually gets pretty subtle. I suggest leaving it out for now.

Ian.