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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx [mailto:win-pv-devel-
> bounces@xxxxxxxxxxxxxxxxxxxx] On Behalf Of Rafal Wojdyla
> Sent: 09 December 2015 14:06
> To: Paul Durrant; win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [win-pv-devel] Advice on evtchn interrupt handling
> 
> 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.
> 

I'd have to check, but that may well mean you only have 2L... I wonder if 
there's a bug in unmask then... possibly the hypercall was not implemented. 
I'll have a look.

  Paul

> >
> >>>> 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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