Re: [Xen-devel] [PATCH] evtchn: don't lose pending state if FIFO event array page is missing

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.10.13 at 16:06, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> When the FIFO-based ABI is in use, if an event is bound when the
> corresponding event array page is missing any attempt to set the event
> pending will lose the event (because there is nowhere to write the
> pending state).
> 
> This wasn't initially considered an issue because guests were expected
> to only bind events once they had expanded the event array, however:
> 
> 1. A domain may start with events already bound (by the toolstack).

But a domain wouldn't start in FIFO mode, would it?

Jan

> 2. The guest does not know what the port number will be until the
>    event is bound (it doesn't know how many already bound events there
>    are), so it does not know how many event array pages are required.
>    This makes it difficult to expand in advanced (the current Linux
>    implementation expands after binding for example).
> 
> To prevent pending events from being lost because there is no array
> page, temporarily store the pending state in evtchn->pending.  When an
> array page is added, use this state to set the port as pending.



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