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

[Gleb Natapov]

On Wed, Mar 14, 2012 at 06:52:07PM +0800, Wen Congyang wrote:
> At 03/14/2012 06:37 PM, Amit Shah Wrote:
> > On (Wed) 14 Mar 2012 [17:53:00], Wen Congyang wrote:
> >> At 03/14/2012 05:24 PM, Avi Kivity Wrote:
> >>> On 03/14/2012 10:29 AM, 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 don't either, copying Amit.
> >>>
> >>>> I start vm like this:
> >>>> qemu ...\
> >>>>    -device virtio-serial \
> >>>>   -chardev socket,path=/tmp/foo,server,nowait,id=foo \
> >>>>   -device virtserialport,chardev=foo,name=port1 ...
> >>>>
> >>>> You said that there are too many channels. Does it mean /tmp/foo is a channel?
> >>>
> >>> Probably.
> >>
> >> Hmm, if we use virtio-serial, the guest kernel writes something into the channel when
> >> the os is panicked. Is it right?
> > 
> > Depends on how you want to use it.  It could be the kernel, or it
> > could be a userspace program which monitors syslogs for panic
> > information and passes on that info to the virtio-serial channel.
> 
> When the kernel is panicked, we cannot use userspace program.
> 
> > 
> >> If so, is this channel visible to guest userspace? If the channle is visible to guest
> >> userspace, the program running in userspace may write the same message to the channel.
> > 
> > Access control is via permissions.  You can have udev scripts assign
> > whatever uid and gid to the port of your interest.  By default, all
> > ports are only accessible to the root user.
> 
> We should also prevent root user writing message to this channel if it is
> used for panicked notification.
> 
Why? Root user can also call panic hypercall if he wishes so.

--
			Gleb.
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