Re: [Qemu-devel] [PATCH 0/2 v3] kvm: notify host when guest panicked

[Daniel P. Berrange]

On Wed, Mar 14, 2012 at 03:21:14PM +0530, Amit Shah wrote:
> On (Wed) 14 Mar 2012 [16:29:50], Wen Congyang wrote:
> > At 03/13/2012 06:47 PM, Avi Kivity Wrote:
> > > On 03/13/2012 11:18 AM, Daniel P. Berrange wrote:
> > >> On Mon, Mar 12, 2012 at 12:33:33PM +0200, Avi Kivity wrote:
> > >>> On 03/12/2012 11:04 AM, Wen Congyang wrote:
> > >>>> Do you have any other comments about this patch?
> > >>>>
> > >>>
> > >>> Not really, but I'm not 100% convinced the patch is worthwhile.  It's
> > >>> likely to only be used by Linux, which has kexec facilities, and you can
> > >>> put talk to management via virtio-serial and describe the crash in more
> > >>> details than a simple hypercall.
> > >>
> > >> As mentioned before, I don't think virtio-serial is a good fit for this.
> > >> We want something that is simple & guaranteed always available. Using
> > >> virtio-serial requires significant setup work on both the host and guest.
> > > 
> > > So what?  It needs to be done anyway for the guest agent.
> > > 
> > >> Many management application won't know to make a vioserial device 
> > >> available
> > >> to all guests they create. 
> > > 
> > > Then they won't know to deal with the panic event either.
> > > 
> > >> Most administrators won't even configure kexec,
> > >> let alone virtio serial on top of it. 
> > > 
> > > It should be done by the OS vendor, not the individual admin.
> > > 
> > >> The hypercall requires zero host
> > >> side config, and zero guest side config, which IMHO is what we need for
> > >> this feature.
> > > 
> > > If it was this one feature, yes.  But we keep getting more and more
> > > features like that and we bloat the hypervisor.  There's a reason we
> > > have a host-to-guest channel, we should use it.
> > > 
> > 
> > I donot know how to use virtio-serial.
> > 
> > I start vm like this:
> > qemu ...\
> >    -device virtio-serial \
> >   -chardev socket,path=/tmp/foo,server,nowait,id=foo \
> >   -device virtserialport,chardev=foo,name=port1 ...
> 
> This is sufficient.  On the host, you can open /tmp/foo using a custom
> program or nc (nc -U /tmp/foo).  On the guest, you can just open
> /dev/virtio-ports/port1 and read/write into it.
> 
> See the following links for more details.
> 
> https://fedoraproject.org/wiki/Features/VirtioSerial#How_To_Test
> http://www.linux-kvm.org/page/Virtio-serial_API
> 
> > You said that there are too many channels. Does it mean /tmp/foo is a 
> > channel?
> 
> You can have several such -device virtserialport.  The -device part
> describes what the guest will see.  The -chardev part ties that to the
> host-side part of the channel.
> 
> /tmp/foo is the host end-point for the channel, in the example above,
> and /dev/virtio-ports/port1 is the guest-side end-point.

If we do choose to use virtio-serial for panics (which I don't think
we should), then we should not expose it in the host filesystem. The
host side should be a virtual chardev backend internal to QEMU, in
the same way that 'spicevmc' is handled.

Regards,
Daniel
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