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

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

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

  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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