linux-kernel - Re: [PATCH] kvm/x86/vmx: report KVM_SYSTEM_EVENT

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Message-ID: <1394433d-528a-06c1-6684-74c4c1dd3081@redhat.com>
Date:   Tue, 28 Mar 2017 14:39:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     rkrcmar@...hat.com, x86@...nel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH] kvm/x86/vmx: report KVM_SYSTEM_EVENT_CRASH on triple
 fault



On 28/03/2017 13:46, Josh Poimboeuf wrote:
> On Tue, Mar 28, 2017 at 03:51:01AM -0400, Paolo Bonzini wrote:
>>
>>> While debugging a kernel issue, I found that QEMU always reboots when an
>>> x86 triple fault occurs, which complicates debugging.  QEMU and libvirt
>>> have a facility for creating a dump when KVM reports
>>> KVM_SYSTEM_EVENT_CRASH.  So change the VMX triple fault handler to do
>>> that.  This gives user space the ability to decide whether to dump,
>>> pause, shutdown, or reboot.
>>
>> You probably want QEMU's -no-reboot option.
>>
>> Triple faults are already reported to userspace with KVM_EXIT_SHUTDOWN,
>> and it's up to userspace to decide what to do with it.  This patch cannot
>> be applied, because there are guests that do a triple-fault intentionally
>> in order to reset the machine.
> 
> Ok.  Any idea how to force libvirt to create a dump?  It has a
> 'coredump-destroy' option, but that only seems to work with 'on_crash':
> 
>   https://libvirt.org/formatdomain.html#elementsEvents

Probably QEMU, when invoked with -no-shutdown -no-reboot, should treat
KVM_EXIT_SHUTDOWN as a panic.  I can have a go at it, but note that QEMU
is now in hard freeze for the next release, so it will take a while.

However you're using libvirt and it doesn't use -no-reboot.

It's probably possible for libvirt to use -no-reboot more often.  The
price would be that if libvirtd crashes and a VM wants to reset, then
the VM gets stuck.

Alternatively, we could generalize -no-shutdown and -no-reboot to
something like:

  -action reset=stop|restart|quit,
          poweroff=stop|quit,
          triple-fault=stop|panic|restart|quit

and teach libvirt about it.  The current semantics map relatively easily
to the new option:

                          | reset       | poweroff   | triple-fault
--------------------------+-------------+------------+-------------------
no option                 | restart     | quit       | restart
-no-shutdown              | restart     | stop       | restart
-no-reboot                | quit        | quit       | quit
-no-shutdown -no-reboot   | stop        | stop       | stop (panic?)

Paolo