Re: [Qemu-devel] Error handling for KVM_GET_DIRTY_LOG

[Paolo Bonzini]

On 16/02/2017 15:51, Janosch Frank wrote:
> 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.

Guest memory problems should not return EFAULT, which corresponds to a
wrong address passed to KVM_GET_DIRTY_LOG.  In fact, EFAULT is probably
the only case where an assertion is warranted---just like you passed a
wrong pointer to KVM_GET_DIRTY_LOG, who knows who else is going to get
that pointer.

ENOENT and EINVAL should not kill the source guest, though they should
terminate migration.  But then I would like to know more about this
case, because they should never happen unless KVMMemoryListener is buggy.

Paolo

> 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
>