Re: [Draft F] Xen on ARM vITS Handling

[Julien Grall <julien.grall@citrix.com>]

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