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

[Keir Fraser <keir.xen@gmail.com>]

On 01/12/2011 18:10, "Tim Deegan" <tim@xen.org> 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.

 -- Keir

> Cheers,
> 
> Tim.