Re: Panic under load when ran under QEMU

[Andrius V <vezhlys%gmail.com@localhost>]

On Fri, Jun 26, 2026 at 1:47 PM ci4ic4 <ci4ic4%proton.me@localhost> wrote:
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> One of my NetBSD nodes started having panics under load - when running a full bob build - recently. It is a VM on a ProxMox server, all updated and setup somewhat non-standard - stuff accumulated over the years and perhaps not following the best in practice advice; the virtual disks live on two ZFS pools backed by fast NVME disks. Anyway, I passed the panic traces to Claude Code, which found a very plausible issue as follows:
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> Confirmed fixed. Summary for the NetBSD PR:
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> **Bug:** `sys/dev/pci/ld_virtio.c` doesn't negotiate `VIRTIO_F_RING_EVENT_IDX`. Under heavy I/O with `iothread=1` on QEMU, completion interrupts flood the handler faster than it drains the used ring → livelock → watchdog panic.
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> **Evidence:** `if_vioif.c:508` negotiates it; `ld_virtio.c` has zero mentions. Block device features at boot show no `EVENT_IDX`; net device shows it.
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> **Workaround:** switch virtio-blk to virtio-scsi (`vioscsi` driver unaffected).
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> **Fix:** add `VIRTIO_F_RING_EVENT_IDX` to `req_features` in `ld_virtio.c` + event-idx update after completion drain, mirroring `if_vioif.c` lines 508/2212/2381.
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> ----
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> I won't be filing a PR as I understand LLM contributions are not accepted, but perhaps somebody might be interested to confirm it. I switched to virtio-scsi and I no longer have the issue.
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> Chavdar
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> Sent with Proton Mail secure email.
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>

Hi,

I think you should file PR nevertheless, since the bug is likely real
(I think they were some recent fixes on ld_virtio relatively recently,
but likely this is a different bug).
NetBSD project doesn't allow LLM generated code, but you can provide
backtrace (if you still have it), describe an issue, and even this
analysis I suppose (with the clear note that it was AI analysed),
I think it was some discussion about using LLM in mailing lists
recently, where policy will be just to clearly state that this text is
AI output.
Don't submit actual code or patches.

Regards,
Andrius V