Re: [Qemu-devel] [PATCH v8] kvm: notify host when the guest is panicked

[Anthony Liguori]

Marcelo Tosatti <address@hidden> writes:

> On Tue, Aug 14, 2012 at 01:53:01PM -0500, Anthony Liguori wrote:
>> Marcelo Tosatti <address@hidden> writes:
>> 
>> > On Tue, Aug 14, 2012 at 05:55:54PM +0300, Yan Vugenfirer wrote:
>> >> 
>> >> On Aug 14, 2012, at 1:42 PM, Jan Kiszka wrote:
>> >> 
>> >> > On 2012-08-14 10:56, Daniel P. Berrange wrote:
>> >> >> On Mon, Aug 13, 2012 at 03:21:32PM -0300, Marcelo Tosatti wrote:
>> >> >>> On Wed, Aug 08, 2012 at 10:43:01AM +0800, Wen Congyang wrote:
>> >> >>>> We can know the guest is panicked when the guest runs on xen.
>> >> >>>> But we do not have such feature on kvm.
>> >> >>>> 
>> >> >>>> Another purpose of this feature is: management app(for example:
>> >> >>>> libvirt) can do auto dump when the guest is panicked. If management
>> >> >>>> app does not do auto dump, the guest's user can do dump by hand if
>> >> >>>> he sees the guest is panicked.
>> >> >>>> 
>> >> >>>> We have three solutions to implement this feature:
>> >> >>>> 1. use vmcall
>> >> >>>> 2. use I/O port
>> >> >>>> 3. use virtio-serial.
>> >> >>>> 
>> >> >>>> We have decided to avoid touching hypervisor. The reason why I choose
>> >> >>>> choose the I/O port is:
>> >> >>>> 1. it is easier to implememt
>> >> >>>> 2. it does not depend any virtual device
>> >> >>>> 3. it can work when starting the kernel
>> >> >>> 
>> >> >>> How about searching for the "Kernel panic - not syncing" string 
>> >> >>> in the guests serial output? Say libvirtd could take an action upon
>> >> >>> that?
>> >> >> 
>> >> >> No, this is not satisfactory. It depends on the guest OS being
>> >> >> configured to use the serial port for console output which we
>> >> >> cannot mandate, since it may well be required for other purposes.
>> >> > 
>> >> Please don't forget Windows guests, there is no console and no "Kernel 
>> >> Panic" string ;)
>> >> 
>> >> What I used for debugging purposes on Windows guest is to register a 
>> >> bugcheck callback in virtio-net driver and write 1 to VIRTIO_PCI_ISR 
>> >> register.
>> >> 
>> >> Yan. 
>> >
>> > Considering whether a "panic-device" should cover other OSes is also \
>
>> > something to consider. Even for Linux, is "panic" the only case which
>> > should be reported via the mechanism? What about oopses without panic? 
>> >
>> > Is the mechanism general enough for supporting new events, etc.
>> 
>> Hi,
>> 
>> I think this discussion is gone of the deep end.
>> 
>> Forget about !x86 platforms.  They have their own way to do this sort of
>> thing.  
>
> The panic function in kernel/panic.c has the following options, which
> appear to be arch independent, on panic:
>
> - reboot 
> - blink

Not sure the semantics of blink but that might be a good place for a
pvops hook.

>
> None are paravirtual interfaces however.
>
>> Think of this feature like a status LED on a motherboard.  These
>> are very common and usually controlled by IO ports.
>> 
>> We're simply reserving a "status LED" for the guest to indicate that it
>> has paniced.  Let's not over engineer this.
>
> My concern is that you end up with state that is dependant on x86.
>
> Subject: [PATCH v8 3/6] add a new runstate: RUN_STATE_GUEST_PANICKED
>
> Having the ability to stop/restart the guest (and even introducing a 
> new VM runstate) is more than a status LED analogy.

I must admit, I don't know why a new runstate is necessary/useful.  The
kernel shouldn't have to care about the difference between a halted guest
and a panicked guest.  That level of information belongs in userspace IMHO.

> Can this new infrastructure be used by other architectures?

I guess I don't understand why the kernel side of this isn't anything
more than a paravirt op hook that does a single outb() with the
remaining logic handled 100% in QEMU.

> Do you consider allowing support for Windows as overengineering?

I don't think there is a way to hook BSOD on Windows so attempting to
engineer something that works with Windows seems odd, no?

Regards,

Anthony Liguori

>
>> Regards,
>> 
>> Anthony Liguori
>> 
>> >
>> >> 
>> >> > Well, we have more than a single serial port, even when leaving
>> >> > virtio-serial aside...
>> >> > 
>> >> > Jan
>> >> > 
>> >> > -- 
>> >> > Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
>> >> > Corporate Competence Center Embedded Linux
>> >> > --
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