Implementation choices, XEN or KVM?

Implementation choices, XEN or KVM?

[Liu John]

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Hello all,

I will do research experiment using hypervisor. But I have not decided the
platform; both XEN and KVM work for me(my work is about MMU, specifically
shadow page tables). Though I prefer XEN, it seems XEN development and
debugging are not easy. For example, every time you add some code in
hypervisor, you have to restart computer.

I appreciate your suggestion.


Thanks
Liu, John

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Re: Implementation choices, XEN or KVM?

[Andrew Cooper]

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On 24/06/11 21:56, Liu John wrote:
> Hello all,
>
> I will do research experiment using hypervisor. But I have not decided
> the platform; both XEN and KVM work for me(my work is about MMU,
> specifically shadow page tables). Though I prefer XEN, it seems XEN
> development and debugging are not easy. For example, every time you
> add some code in hypervisor, you have to restart computer.
>
> I appreciate your suggestion.
>
>
> Thanks
> Liu, John 

I am curious to know under what circumstances you think it is, or should
be possible to make hypervisor code changes without rebooting?

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com


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Re: Implementation choices, XEN or KVM?

[Tim Deegan]

At 16:56 -0400 on 24 Jun (1308934602), Liu John wrote:
> I will do research experiment using hypervisor. But I have not decided the
> platform; both XEN and KVM work for me(my work is about MMU, specifically
> shadow page tables). Though I prefer XEN, it seems XEN development and
> debugging are not easy. For example, every time you add some code in
> hypervisor, you have to restart computer.

That's true.   However I usually find that when I make a mistake in
hypervisor code the computer needs to be rebooted anyway. :) 
AIUI Xen can run under Qemu so the reboot time is not that long. 

Tim.

-- 
Tim Deegan <Tim.Deegan@citrix.com>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

Re: Implementation choices, XEN or KVM?

[George Dunlap]

On Mon, Jun 27, 2011 at 8:50 AM, Andrew Cooper
<andrew.cooper3@citrix.com> wrote:
> I am curious to know under what circumstances you think it is, or should be
> possible to make hypervisor code changes without rebooting?

Perhaps the following works in KVM?

$ make kvm.ko
$ sudo rmmod kvm
$ sudo insmod kvm.ko

If it's stuff related only to the interface seen by VMs, and not to
the interaction w/ the hardware, it's probably pretty convenient. :-)

 -George

RE: Implementation choices, XEN or KVM?

[James Harper]

> 
> At 16:56 -0400 on 24 Jun (1308934602), Liu John wrote:
> > I will do research experiment using hypervisor. But I have not
decided the
> > platform; both XEN and KVM work for me(my work is about MMU,
specifically
> > shadow page tables). Though I prefer XEN, it seems XEN development
and
> > debugging are not easy. For example, every time you add some code in
> > hypervisor, you have to restart computer.
> 
> That's true.   However I usually find that when I make a mistake in
> hypervisor code the computer needs to be rebooted anyway. :)
> AIUI Xen can run under Qemu so the reboot time is not that long.
> 

Just out of curiosity, can xen be run inside xen yet? And does it make
debugging any easier?

James

Re: Implementation choices, XEN or KVM?

[Tim Deegan]

At 20:25 +1000 on 27 Jun (1309206300), James Harper wrote:
> > 
> > At 16:56 -0400 on 24 Jun (1308934602), Liu John wrote:
> > > I will do research experiment using hypervisor. But I have not
> decided the
> > > platform; both XEN and KVM work for me(my work is about MMU,
> specifically
> > > shadow page tables). Though I prefer XEN, it seems XEN development
> and
> > > debugging are not easy. For example, every time you add some code in
> > > hypervisor, you have to restart computer.
> > 
> > That's true.   However I usually find that when I make a mistake in
> > hypervisor code the computer needs to be rebooted anyway. :)
> > AIUI Xen can run under Qemu so the reboot time is not that long.
> > 
> 
> Just out of curiosity, can xen be run inside xen yet? And does it make
> debugging any easier?

It can, as a HVM guest, (although I've been having trouble booting
latest xen-unstable under itself; running Xen 4.0 under latest unstable
seems to work).  We've just had support added for running HVM guests
inside nested Xen too.

I don't use it since I'm often working on hardware-specific or
performance-related issues, neither of which debugs very well in a VM.
For general development I keep a tight hold of one or two test-boxes
with fast BIOS POST times. :)

Tim.

-- 
Tim Deegan <Tim.Deegan@citrix.com>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

Re: Implementation choices, XEN or KVM?

[Wei Liu]

On Mon, Jun 27, 2011 at 4:46 PM, Tim Deegan <Tim.Deegan@citrix.com> wrote:
> At 16:56 -0400 on 24 Jun (1308934602), Liu John wrote:
>> I will do research experiment using hypervisor. But I have not decided the
>> platform; both XEN and KVM work for me(my work is about MMU, specifically
>> shadow page tables). Though I prefer XEN, it seems XEN development and
>> debugging are not easy. For example, every time you add some code in
>> hypervisor, you have to restart computer.
>
> That's true.   However I usually find that when I make a mistake in
> hypervisor code the computer needs to be rebooted anyway. :)
> AIUI Xen can run under Qemu so the reboot time is not that long.
>

Just out of my curiosity.

Does Xen inside QEMU supports HVM guest?

Wei.

Re: Implementation choices, XEN or KVM?

[Tim Deegan]

At 18:30 +0800 on 27 Jun (1309199414), Wei Liu wrote:
> On Mon, Jun 27, 2011 at 4:46 PM, Tim Deegan <Tim.Deegan@citrix.com> wrote:
> > At 16:56 -0400 on 24 Jun (1308934602), Liu John wrote:
> >> I will do research experiment using hypervisor. But I have not decided the
> >> platform; both XEN and KVM work for me(my work is about MMU, specifically
> >> shadow page tables). Though I prefer XEN, it seems XEN development and
> >> debugging are not easy. For example, every time you add some code in
> >> hypervisor, you have to restart computer.
> >
> > That's true.   However I usually find that when I make a mistake in
> > hypervisor code the computer needs to be rebooted anyway. :)
> > AIUI Xen can run under Qemu so the reboot time is not that long.
> >
> 
> Just out of my curiosity.
> 
> Does Xen inside QEMU supports HVM guest?

I haven't tried it.  ISTR seeing patches for SVM support, at least, on
qemu-devel a while back.  So: maybe. :)

Tim.

-- 
Tim Deegan <Tim.Deegan@citrix.com>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

Re: Implementation choices, XEN or KVM?

[Andre Przywara]

On 06/27/2011 12:40 PM, Tim Deegan wrote:
> At 18:30 +0800 on 27 Jun (1309199414), Wei Liu wrote:
>> On Mon, Jun 27, 2011 at 4:46 PM, Tim Deegan<Tim.Deegan@citrix.com>  wrote:
>>> At 16:56 -0400 on 24 Jun (1308934602), Liu John wrote:
>>>> I will do research experiment using hypervisor. But I have not decided the
>>>> platform; both XEN and KVM work for me(my work is about MMU, specifically
>>>> shadow page tables). Though I prefer XEN, it seems XEN development and
>>>> debugging are not easy. For example, every time you add some code in
>>>> hypervisor, you have to restart computer.
>>>
>>> That's true.   However I usually find that when I make a mistake in
>>> hypervisor code the computer needs to be rebooted anyway. :)
>>> AIUI Xen can run under Qemu so the reboot time is not that long.
>>>
>>
>> Just out of my curiosity.
>>
>> Does Xen inside QEMU supports HVM guest?
>
> I haven't tried it.  ISTR seeing patches for SVM support, at least, on
> qemu-devel a while back.  So: maybe. :)

As Tim said, QEMU implements some basic SVM emulation. But as this is 
quite old and nobody seemed to care, it is currently broken, AFAICT. At 
best it is not complete anyway and so probably unusable.

But you can use the nested SVM support in KVM. Though the primary test 
vehicle is KVM on KVM ;-), we also tests Xen as a guest from time to 
time and it worked. So just use QEMU and say --enable-kvm or use 
qemu-kvm on an AMD machine and you can run Xen with HVM guests inside KVM.

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany