Re: [Qemu-devel] Linux kernel polling for QEMU

[Paolo Bonzini]

On 29/11/2016 14:24, Fam Zheng wrote:
> On Tue, 11/29 12:17, Paolo Bonzini wrote:
>>
>>
>> On 29/11/2016 11:32, Fam Zheng wrote:
>>>
>>> The kernel change will be a new prctl operation (should it be a different
>>> syscall to extend?) to register a new type of eventfd called "idle eventfd":
>>>
>>>     prctl(PR_ADD_IDLE_EVENTFD, int eventfd);
>>>     prctl(PR_DEL_IDLE_EVENTFD, int eventfd);
>>>
>>> It will be notified by kernel each time when the thread's local core has no
>>> runnable threads (i.e., entering idle state).
>>>
>>> QEMU can then add this eventfd to its event loop when it has events to 
>>> poll, and
>>> watch virtqueue/linux-aio memory from userspace in the fd handlers.  
>>> Effectiely,
>>> if a ppoll() would have blocked because there are no new events, it could 
>>> now
>>> return immediately because of idle_eventfd events, and do the idle polling.
>>
>> This has two issues:
>>
>> * it only reports the leading edge of single_task_running().  Is it also
>> useful to stop polling on the trailing edge?
> 
> QEMU can clear the eventfd right after event firing so I don't think it is
> necessary.

Yes, but how would QEMU know that the eventfd has fired?  It would be
very expensive to read the eventfd on each iteration of polling.

Paolo

>> * it still needs a system call before polling is entered.  Ideally, QEMU
>> could run without any system call while in polling mode.
>>
>> Another possibility is to add a system call for single_task_running().
>> It should be simple enough that you can implement it in the vDSO and
>> avoid a context switch.  There are convenient hooking points in
>> add_nr_running and sub_nr_running.
> 
> That sounds good!
> 
> Fam
>