Apparently this was first noticed with 4.14, but more recently I've been able to reproduce the issue:
https://bugs.debian.org/988477 The original observation features MD-RAID1 using a pair of Samsung SATA-attached flash devices. The main line shows up in `xl dmesg`: (XEN) AMD-Vi: IO_PAGE_FAULT: DDDD:bb:dd.f d0 addr ffffff???????000 flags 0x8 I Where the device points at the SATA controller. I've ended up reproducing this with some noticable differences. A major goal of RAID is to have different devices fail at different times. Hence my initial run had a Samsung device plus a device from another reputable flash manufacturer. I initially noticed this due to messages in domain 0's dmesg about errors from the SATA device. Wasn't until rather later that I noticed the IOMMU warnings in Xen's dmesg (perhaps post-domain 0 messages should be duplicated into domain 0's dmesg?). All of the failures consistently pointed at the Samsung device. Due to the expectation it would fail first (lower quality offering with lesser guarantees), I proceeded to replace it with a NVMe device. With some monitoring I discovered the NVMe device was now triggering IOMMU errors, though not nearly as many as the Samsung SATA device did. As such looks like AMD-Vi plus MD-RAID1 appears to be exposing some sort of IOMMU issue with Xen. All I can do is offer speculation about the underlying cause. There does seem to be a pattern of higher-performance flash storage devices being more severely effected. I was speculating about the issue being the MD-RAID1 driver abusing Linux's DMA infrastructure in some fashion. Upon further consideration, I'm wondering if this is perhaps a latency issue. I imagine there is some sort of flush after the IOMMU tables are modified. Perhaps the Samsung SATA (and all NVMe) devices were trying to execute commands before reloading the IOMMU tables is complete. -- (\___(\___(\______ --=> 8-) EHM <=-- ______/)___/)___/) \BS ( | ehem+sig...@m5p.com PGP 87145445 | ) / \_CS\ | _____ -O #include <stddisclaimer.h> O- _____ | / _/ 8A19\___\_|_/58D2 7E3D DDF4 7BA6 <-PGP-> 41D1 B375 37D0 8714\_|_/___/5445
