Re: [Xen-devel] [Draft F] Xen on ARM vITS Handling

[Julien Grall <julien.grall@xxxxxxxxxx>]

On 12/06/2015 08:55, Ian Campbell wrote:
> 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.
Would it be possible to do it with re-doing the write to the GITS_CWRITER?

Regards,

--
Julien Grall

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