Date:	Mon, 23 Jul 2012 01:50:56 +0200
From:	Sasha Levin <levinsasha928@...il.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Wen Congyang <wency@...fujitsu.com>,
	Gleb Natapov <gleb@...hat.com>, kvm list <kvm@...r.kernel.org>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	qemu-devel <qemu-devel@...gnu.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Avi Kivity <avi@...hat.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	rusty@...tcorp.com.au
Subject: Re: [Qemu-devel] [PATCH v7.5] kvm: notify host when the guest is
 panicked

On 07/23/2012 12:36 AM, Anthony Liguori wrote:
> Sasha Levin <levinsasha928@...il.com> writes:
> 
>> On 07/22/2012 09:14 PM, Anthony Liguori wrote:
>>> Sasha Levin <levinsasha928@...il.com> writes:
>>>
>>>> On 07/21/2012 10:44 AM, 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
>>>>
>>>> Was the option of implementing a virtio-watchdog driver considered?
>>>>
>>>> You're basically re-implementing a watchdog, a guest-host interface and a set of protocols for guest-host communications.
>>>>
>>>> Why can't we re-use everything we have now, push a virtio watchdog
>>>> driver into drivers/watchdog/, and gain a more complete solution to
>>>> detecting hangs inside the guest.
>>>
>>> The purpose of virtio is not to reinvent every possible type of device.
>>> There are plenty of hardware watchdogs that are very suitable to be used
>>> for this purpose.  QEMU implements quite a few already.
>>>
>>> Watchdogs are not performance sensitive so there's no point in using
>>> virtio.
>>
>> The issue here is not performance, but the adding of a brand new
>> guest-host interface.
> 
> We have:
> 
> 1) Virtio--this is our preferred PV interface.  It needs PCI to be fully
> initialized and probably will live as a module.
> 
> 2) Hypercalls--this a secondary PV interface but is available very
> early.  It's terminated in kvm.ko which means it can only operate on
> things that are logically part of the CPU and/or APIC complex.
> 
> This patch introduces a third interface which is available early like
> hypercalls but not necessarily terminated in kvm.ko.  That means it can
> have a broader scope in functionality than (2).
> 
> We could just as well use a hypercall and have multiple commands issued
> to that hypercall as a convention and add a new exit type to KVM that
> sent that specific hypercall to userspace for processing.
> 
> But a PIO operation already has this behavior and requires no changes to kvm.ko.

I don't dispute that there may be a need for another guest-host interface, but this patch can basically be called "kvm: notify host when the guest is panicked, oh, btw, and add a brand new undocumented interface"

The new interface should at least come in it's own patch, with documentation.
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