linux-kernel - Re: [PATCH] KVM: Add wrapper script around QEMU to test kernels

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Date:	Tue, 08 Nov 2011 16:04:29 +0100
From:	Jan Kiszka <jan.kiszka@....de>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Avi Kivity <avi@...hat.com>, Alexander Graf <agraf@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org list" <kvm@...r.kernel.org>,
	qemu-devel Developers <qemu-devel@...gnu.org>,
	Pekka Enberg <penberg@...nel.org>,
	Am?rico Wang <xiyou.wangcong@...il.com>,
	Blue Swirl <blauwirbel@...il.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH] KVM: Add wrapper script around QEMU to test kernels

On 2011-11-08 15:52, Christoph Hellwig wrote:
> On Tue, Nov 08, 2011 at 04:41:40PM +0200, Avi Kivity wrote:
>> On 11/06/2011 03:35 AM, Alexander Graf wrote:
>>> To quickly get going, just execute the following as user:
>>>
>>>     $ ./Documentation/run-qemu.sh -r / -a init=/bin/bash
>>>
>>> This will drop you into a shell on your rootfs.
>>>
>>
>> Doesn't work on Fedora 15.  F15's qemu-kvm doesn't have -machine or
>> -virtfs.  Even qemu.git on F15 won't build virtfs since xattr.h
>> detection is broken (patch posted).
> 
> Nevermind that running virtfs as a rootfs is a really dumb idea.  You
> do now want to run a VM that has a rootfs that gets changed all the
> time behind your back.
> 
> Running qemu -snapshot on the actual root block device is the only
> safe way to reuse the host installation, although it gets a bit
> complicated if people have multiple devices mounted into the namespace.

I thought about this while hacking a slide on this topic: It's clumsy
(compared to -snapshot - my favorite one as well), but you could use
some snapshot on the host fs. Or a union fs (if we had  an official one)
with the write layer directed to some tmpfs area.

But what we likely rather want (as it would work without privileges) is
built-in write redirection for virtfs. Not an expert on this, but I
guess that will have to solve the same problems an in-kernel union fs
solution faces, no?

Jan


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