[Qemu-devel] Error handling for KVM_GET_DIRTY_LOG

[Janosch Frank]

While trying to fix a bug in the s390 migration code, I noticed that
QEMU ignores practically all errors returned from that VM ioctl. QEMU
behaves as specified in the KVM api and only processes -1 (-EPERM) as an
error.

Unfortunately the documentation is wrong/old and KVM may return -EFAULT,
-EINVAL, -ENOTSUPP (BookE) and -ENOENT. This bugs me, as I found a case
where I want to return -EFAULT because of guest memory problems and QEMU
will still happily migrate the VM.

I currently don't see a reason why we continue to migrate on EFAULT and
EINVAL. But returning -error from kvm_physical_sync_dirty_bitmap might
also a bit hard, as it kills QEMU.

Do we want to fix this and if, how do we want it done?
If not we at least have a definitive mail to point to when the next one
comes around. I also have a KVM patch to update the api documentation if
wanted (maybe we should dust that off a bit anyhow).


This has been brought up in 2009 [1] the first time and was more or less
fixed and then reverted in 2014 [2].

The reason in [1] was that PPC hadn't settled yet on a valid return code.

In [2] it was too close to the v2 to handle it properly.


[1] https://lists.nongnu.org/archive/html/qemu-devel/2009-07/msg01772.html

[2] https://lists.nongnu.org/archive/html/qemu-devel/2014-04/msg01993.html


Cheers,
Janosch