Re: [Qemu-devel] [PATCH v11] kvm: notify host when the guest is panicked

[Marcelo Tosatti]

On Tue, Oct 30, 2012 at 10:30:02PM -0400, Sasha Levin wrote:
> On Tue, Oct 30, 2012 at 9:48 PM, Wen Congyang <address@hidden> wrote:
> > At 10/31/2012 09:12 AM, Marcelo Tosatti Wrote:
> >> It has been asked earlier why a simple virtio device is not usable
> >> for this (with no response IIRC).
> >
> > 1. We can't use virtio device when the kernel is booting.
> 
> So the issue here is the small window between the point the guest
> becomes "self aware" and to the point virtio drivers are loaded,
> right?
> 
> I agree that if something happens during that interval, a
> "virtio-notifier" driver won't catch that, but anything beyond that is
> better done with a virtio driver, so how is the generic infrastructure
> added in this patch useful to anything beyond detecting panics in that
> initial interval?

Asked earlier about quantification of panics in that window (i doubt
early panics are that significant for this usecase). netconsole has
the same issue:

"This module logs kernel printk messages over UDP allowing debugging of
problem where disk logging fails and serial consoles are impractical.

It can be used either built-in or as a module. As a built-in,
netconsole initializes immediately after NIC cards and will bring up
the specified interface as soon as possible. While this doesn't allow
capture of early kernel panics, it does capture most of the boot
process."

> > 2. The virtio's driver can be built as a module, and if it is not loaded
> >    and the kernel is panicked, there is no way to notify the host.
> 
> Even if the suggested virtio-notifier driver is built as a module, it
> would get auto-loaded when the guest is booting, so I'm not sure about
> this point?

> > 3. I/O port is more reliable than virtio device.
> >    If virtio's driver has some bug, and it cause kernel panicked, we can't
> >    use it. The I/O port is more reliable because it only depends on notifier
> >    chain(If we use virtio device, it also depends on notifier chain).
> 
> This is like suggesting that we let KVM emulate virtio-blk on it's
> own, parallel to the virtio implementation, so that even if there's a
> problem with virtio-blk, KVM can emulate a virtio-blk on it's own.
> 
> Furthermore, why stop at virtio? What if the KVM code has a bug and it
> doesn't pass IO properly? Or the x86 code? we still want panic
> notifications if that happens...