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

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Date:	Wed, 14 Mar 2012 15:25:52 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Wen Congyang <wency@...fujitsu.com>,
	"Daniel P. Berrange" <berrange@...hat.com>,
	kvm list <kvm@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Amit Shah <amit.shah@...hat.com>
Subject: Re: [PATCH 0/2 v3] kvm: notify host when guest panicked

On Wed, Mar 14, 2012 at 03:16:05PM +0200, Avi Kivity wrote:
> On 03/14/2012 03:14 PM, Gleb Natapov wrote:
> > On Wed, Mar 14, 2012 at 03:07:46PM +0200, Avi Kivity wrote:
> > > On 03/14/2012 01:11 PM, Wen Congyang wrote:
> > > > > 
> > > > > I don't think we want to use the driver.  Instead, have a small piece of
> > > > > code that resets the device and pushes out a string (the panic message?)
> > > > > without any interrupts etc.
> > > > > 
> > > > > It's still going to be less reliable than a hypercall, I agree.
> > > >
> > > > Do you still want to use complicated and less reliable way?
> > > 
> > > Are you willing to try it out and see how complicated it really is?
> > > 
> > > While it's more complicated, it's also more flexible.  You can
> > > communicate the panic message, whether the guest is attempting a kdump
> > > and its own recovery or whether it wants the host to do it, etc., you
> > > can communicate less severe failures like oopses.
> > > 
> > hypercall can take arguments to achieve the same.
> 
> It has to be designed in advance; and every time we notice something's
> missing we have to update the host kernel.
> 

We and in the designed stage now. Not to late to design something flexible
:) Panic hypercall can take GPA of a buffer where host puts panic info
as a parameter.  This buffer can be read by QEMU and passed to management.

> > > > I think the other ones prefer to touch the hypervisor.
> > > 
> > > I understand the sentiment.  Your patches are simple and easy.  But my
> > > feeling is that the kernel has become too complicated already and I'm
> > > looking for ways to limit changes.
> > > 
> > Using virtio-serial will not reduce kernel complexity. Quite contrary
> > since code that will use virtio-serial will be more complicated.
> 
> The host kernel is unmodified though.
> 
Yes, this is trade-off between complexity in hypervisor and a guest
kernel. But in the end we use the same kernel for both.

--
			Gleb.
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