Re: [PATCH 2/2] KVM: Create debugfs dir and stat files for each VM

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 11/26/2015 05:17 PM, Tyler Baker wrote:
> Hi Christian,
> 
> The kernelci.org bot recently has been reporting kvm guest boot
> failures[1] on various arm64 platforms in next-20151126. The bot
> bisected[2] the failures to the commit in -next titled "KVM: Create
> debugfs dir and stat files for each VM". I confirmed by reverting this
> commit on top of next-20151126 it resolves the boot issue.
> 
> In this test case the host and guest are booted with the same kernel.
> The host is booted over nfs, installs qemu (qemu-system arm64 2.4.0),
> and launches a guest. The host is booting fine, but when the guest is
> launched it errors with "Failed to retrieve host CPU features!". I
> checked the host logs, and found an "Unable to handle kernel paging
> request" splat[3] which occurs when the guest is attempting to start.
> 
> I scanned the patch in question but nothing obvious jumped out at me,
> any thoughts?

Not really.
Do you have processing running that do read the files in /sys/kernel/debug/kvm/* ?

If I read the arm oops message correctly it oopsed inside 
__srcu_read_lock. there is actually nothing in there that can oops, 
except the access to the preempt count. I am just guessing right now,
but maybe the preempt variable is no longer available (as the process
is gone). As long as a debugfs file is open, we hold a reference to 
the kvm, which holds a reference to the mm, so the mm might be killed
after the process. But this is supposed to work, so maybe its something
different. An objdump of __srcu_read_lock might help.

I will drop it from my tree until we understand the problem

Christian