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

[Tim Deegan <tim@xxxxxxx>]

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?

Cheers,

Tim.

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