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

[Amit Shah]

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.

                Amit