linux-kernel - Re: Qemu KVM thread spins at 100% CPU usage on scsi hot-unplug (kernel 6.6.8 guest)

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Message-ID: <20231228131802.GA21994@wunner.de>
Date: Thu, 28 Dec 2023 14:18:02 +0100
From: Lukas Wunner <lukas@...ner.de>
To: Steven Haigh <netwiz@....id.au>
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	f.ebner@...xmox.com
Subject: Re: Qemu KVM thread spins at 100% CPU usage on scsi hot-unplug
 (kernel 6.6.8 guest)

On Thu, Dec 28, 2023 at 01:03:10PM +1100, Steven Haigh wrote:
> At some point in kernel 6.6.x, SCSI hotplug in qemu VMs broke. This was
> mostly fixed in the following commit to release 6.6.8:
> 	commit 5cc8d88a1b94b900fd74abda744c29ff5845430b
> 	Author: Bjorn Helgaas <bhelgaas@...gle.com>
> 	Date:   Thu Dec 14 09:08:56 2023 -0600
> 	Revert "PCI: acpiphp: Reassign resources on bridge if necessary"
> 
> After this commit, the SCSI block device is hotplugged correctly, and a device node as /dev/sdX appears within the qemu VM.
> 
> New problem:
> 
> When the same SCSI block device is hot-unplugged, the QEMU KVM process will
> spin at 100% CPU usage. The guest shows no CPU being used via top, but the
> host will continue to spin in the KVM thread until the VM is rebooted.

Find out the PID of the qemu process on the host, then cat /proc/$PID/stack
to see where the CPU time is spent.