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

[David Vrabel <david.vrabel@citrix.com>]

On 18/10/13 15:41, Jan Beulich wrote:
>>>> On 18.10.13 at 16:06, David Vrabel <david.vrabel@citrix.com> wrote:
>> From: David Vrabel <david.vrabel@citrix.com>
>>
>> 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?

Yes, guests always start in 2-level mode, but if the guest doesn't
unmask the events they should still be pending after switching to FIFO
mode so must copy the pending bits from the 2-level array and set them
pending in the into the FIFO event array (when the pages turn up).

David

>> 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.