Re: [win-pv-devel] Advice on evtchn interrupt handling

[RafaÅ WojdyÅa <omeg@xxxxxxxxxxxxxxxxxxxxxx>]

On 12/09/2015 02:02 PM, Paul Durrant wrote:
>> -----Original Message-----
>> From: RafaÅ WojdyÅa [mailto:omeg@xxxxxxxxxxxxxxxxxxxxxx]
>> Sent: 09 December 2015 11:08
>> To: Paul Durrant; win-pv-devel@xxxxxxxxxxxxxxxxxxxx
>> Subject: Re: [win-pv-devel] Advice on evtchn interrupt handling
>>
>> On 2015-12-09 11:44, Paul Durrant wrote:
>>>> -----Original Message-----
>>>> From: win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx [mailto:win-pv-devel-
>>>> bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf Of Rafal Wojdyla
>>>> Sent: 09 December 2015 10:19
>>>> To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
>>>> Subject: [win-pv-devel] Advice on evtchn interrupt handling
>>>>
>>>> Hi,
>>>>
>>>> I found an issue in my evtchn handling in xeniface that I'm not sure how
>>>> to properly fix. Namely, the interrupt callback that's assigned for a
>>>> particular channel can be called before its queued DPC (that signals the
>>>> caller-provided event) finishes. In such case the interrupt callback
>>>> fails to insert the DPC again and an event is lost, potentially causing
>>>> a vchan "deadlock" down the way.
>>>>
>>>
>>> Hi Rafal,
>>>
>>> That shouldn't be the case. Windows de-queues a DPC before running for
>> precisely this kind of reason. I.e. just because your DPC is running, it 
>> should
>> not stop another from being queued. Are you definitely seeing a failure to
>> queue even when no DPC is actually queued? What you may actually be
>> getting caught by is evtchn auto-masking. If you have enabled auto-masking
>> then you need to explicitly unmask at the end of your DPC. (The transmitter
>> and receiver ring DPCs in XENVIF are a good example of this).
>>>
>>> Cheers,
>>>
>>>   Paul
>>>
>> Here's the relevant bit:
>> http://xenbits.xen.org/gitweb/?p=pvdrivers/win/xeniface.git;a=blob;f=src/x
>> eniface/ioctl_evtchn.c;h=9c5af19e00a91a08f68da0c630709cbaa6e8413d;hb=
>> HEAD#l66
>>
>> I check the result of KeInsertQueueDpc() and I definitely see it fail
>> sometimes, even if the HVM only has 1 vcpu. My DPC unmasks the channel
>> after setting an event so this shouldn't be the reason.
>>
> 
> The article at https://www.osronline.com/article.cfm?article=529 is worth 
> reading. The comment labelled "DPC Reentrancy" points out that DPCs are 
> de-queued before being executed. So really you should not see a queueing 
> failure unless either then unmasking is not working correctly or there's a 
> bug in Windows itself.

Yeah, I've read that before and was wondering whether I'm missing
something that would explain the behavior I see.

> Are you running on a Xen with FIFO events or is it old enough to only be 2L?
> 
>   Paul

We're using 4.4.3 right now.

> 
>>>> Any advice on how to approach this? The interrupt callback runs at
>>>> HIGH_LEVEL which is an issue when it comes to synchronization with
>> some
>>>> other code that could take care of such "pending" events.
>>>>
>>>> Also I wonder if the kernel differentiates DPCs only by their KDPC
>>>> address, or also by arguments passed to KeInsertQueueDpc()?
>>>>

-- 
RafaÅ WojdyÅa
Qubes Tools for Windows developer
https://www.qubes-os.org/

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