Re: [Xen-devel] [PATCH 2 of 5] Improve ring management for memory events. Do not lose guest events

["Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx>]

> On 01/12/2011 18:10, "Tim Deegan" <tim@xxxxxxx> wrote:
>
>> At 16:55 -0500 on 29 Nov (1322585711), Andres Lagar-Cavilla wrote:
>>> The memevent code currently has a mechanism for reserving space in the
>>> ring
>>> before putting an event, but each caller must individually ensure that
>>> the
>>> vCPUs are correctly paused if no space is available.
>>>
>>> This fixes that issue by reversing the semantics: we ensure that enough
>>> space
>>> is always left for one event per vCPU in the ring.  If, after putting
>>> the
>>> current request, this constraint will be violated by the current vCPU
>>> when putting putting another request in the ring, we pause the vCPU.
>>
>> What about operations that touch more than one page of guest memory?
>> (E.g., pagetable walks, emulated faults and task switches).  Can't they
>> still fill up the ring?
>>
>> IIRC there are still cases where we need wait-queues anyway (when we hit
>> a paged-out page after an non-idempotent action has already been
>> taken).  Is the purpose of this change just to reduce the number of
>> wait-queue uses or do you think you can do without them entirely?
>
> It's definitely not possible to do without entirely. I'm pretty sure
> Andres
> agrees at least that far.
I do agree that far :)
Andres

>
>  -- Keir
>
>> Cheers,
>>
>> Tim.
>
>
>



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