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Locked DMA mapping while invalidating mapcache

  • To: "win-pv-devel@xxxxxxxxxxxxxxxxxxxx" <win-pv-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Rafał Wojdyła <omeg@xxxxxxxxxxxxxxxxxxxxxx>
  • Date: Wed, 8 Jan 2025 14:31:15 +0100
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  • Delivery-date: Wed, 08 Jan 2025 13:31:31 +0000
  • Feedback-id: i409c4082:Fastmail
  • List-id: Developer list for the Windows PV Drivers subproject <win-pv-devel.lists.xenproject.org>
Hi,
I've been experiencing guest freezes recently that seem to be related to guest memory page sharing. Guest is completely frozen, I don't see anything abnormal via a kernel debugger, but I see something like this in the qemu log:
[2025-01-08 13:44:35] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x7780775fe430 is present [2025-01-08 13:44:36] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x7780775fe430 is present [2025-01-08 13:44:36] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x7780775fe430 is present [2025-01-08 13:44:39] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x7780775fe430 is present [2025-01-08 13:44:39] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x7780775fe430 is present [2025-01-08 13:44:40] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x7780775fe430 is present [2025-01-08 13:44:48] Locked DMA mapping while invalidating mapcache! 0000000000000efe -> 0x778081b23d50 is present [2025-01-08 13:44:48] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x77808253b430 is present [2025-01-08 13:44:48] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x77808253b430 is present [2025-01-08 13:44:48] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x77808253b430 is present [2025-01-08 13:44:48] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x77808253b430 is present [2025-01-08 13:44:48] Locked DMA mapping while invalidating mapcache! 0000000000000efd -> 0x77808253b430 is present 

...and a lot more repeats of this line.

We use Xen 4.17.5 with qemu 9.0.2 in a stubdomain.
Freezes don't occur if xeniface is installed but no process uses the memory grant APIs. What's weird is that I've started encountering these issues only recently without any changes in driver's code. My guess is that the page sharing code in xeniface is violating some hypervisor or qemu assumption, but why this only manifested now I have no idea. I'm reviewing the code but if you have any ideas on what might be going on, please share :) 

--
Rafał Wojdyła
Invisible Things Lab

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