VMs on netbooks

OK, this post is about virtual machines in the JVM sense, not in the hypervisor sense. (The lines are getting a little blurry these days though.)

Back in the day, I once installed Windows NT on an RS/6000 (PowerPC) just to play with it. (It's funny how obsolete/impractical technology seemed so interesting back then, and these days it boggles my mind how anybody could care about Haiku, Amiga, etc.) So Windows NT: it installed OK, I started it up, and ran IE 2.0. That sucked (even at the time), but there were no updates for it. I ran the bundled Pinball game. That was the end of the story, because there was no ISV support. Just porting the kernel wasn't enough: an x86 WinNT application still couldn't run on PowerPC WinNT. The same fate could befall ARM netbooks (ahem "smartbooks").

This post suggests that .NET could be the answer. It starts by assuming that ARM netbooks will be common (a question on which the jury is still out), and then assumes Microsoft will somehow want to participate in that ecosystem (probably a safe bet: look at Windows ME on phones). Port some Windows kernel to ARM netbooks, provide a .NET runtime, and then just run .NET applications -- never worry about needing native ARM Windows binaries.

That has an existing parallel of course: J2ME on mobile phones. As a consumer, I'd call that a success. I love that I can download a random Java application and not worry about if the creator has built it for N phone vendors x M models. I'm sure J2ME has its limits, but it has made my life better.

And of course Google is walking down the same path with Dalvik. The cool thing about Java/Dalvik/.NET that it might just allow another processor architecture to compete with Intel without the legacy software issue. It will be interesting to see if Intel eventually enables Java on Moblin.

With Intel investing so many resources not just into the Linux kernel, but now into a full mobile Linux distribution (complete with UI), maybe Microsoft will annoy them right back by enabling ARM netbooks. You know both of them have to be looking to embedded systems for growth.

Anyways, I'd only buy a netbook that runs Linux. ;) I've heard good things about the ARM instruction set...

Oracle buys another hypervisor?!

Oracle made headlines for two acqusitions in the past month, Sun and Virtual Iron. By my count, that makes them the proud owners of no fewer than four x86 hypervisors: Oracle VM, Sun's xVM Server, Sun's xVM Desktop (a.k.a. VirtualBox), and Virtual Iron. (I've never really understood why Sun had two.) All but VirtualBox are based on Xen.

Even with this surprising success in the game of "collect the Xen implementations," that still leaves at least Red Hat, Novell, and of course Citrix itself offering competing Xen solutions. I'll admit the Unix wars predate me, but that seems like an impressive degree of fragmentation. Still, Red Hat has announced plans to abandon Xen for KVM, and even Novell included KVM as a tech preview in SLES11. There's no question that the number of Xen-based products is about to significantly shrink.

I've even seen one person speculate that the cost of maintaining Xen is so high, that with Red Hat pulling out, Oracle must have been worried about strengthening their Xen development capabilities.

What's worse though is that each of those Oracle hypervisors has its own management stack. Systems management is one of those areas that sounds really easy, but in practice never is. Management software contains so many layers that it's hard to find anybody who actually understands the end-to-end details of a particular code path. You need translation layers for components that are too old, too new, or fit at a different layer, or written for a different management stack. In this case, you might find one management stack built on xm, another built on libvirt, another built on CIM (and "enterprise frameworks" are a whole other world of complexity). Do they use OVF for image files? Should they? Every design question has tradeoffs and requires serious consideration.

Speaking from experience at a large company, I expect there will be at least 6 months of churn while architects furiously scribe presentations, rearrange block diagrams, create hypothetical development sizings, establish migration plans for legacy customers, escalate issues to management (which will be sorting itself out too), find and get input from related organizations ("how does this affect our relationship with VMware?"), and in general figure out what they're doing. After all the dust has settled, they'll still need to write the code.

Will the eventual result of this consolidation be a stronger Xen ecosystem a few years from now? To be honest, I couldn't care less... but it could be worth the cost of a bag of popcorn.

WAN optimization

I've wondered how "WAN optimization" magic works, and I just came across a page to explain it in (a little) more detail than marketing. I hadn't heard of it until a couple years ago, when some Cisco people mentioned that their WAASrouters embed KVM for virtualization.

Why would they do that? Because if your branch office uses an Active Directory server in your centralized data center, and your WAN link dies, work at the branch office ceases. From what I understand, Cisco's WAAS routers run an Active Directory server inside a virtual machine on the router itself, to mitigate that problem. A little googling reveals that similar approaches may be taken by their competition, 3Com and Riverbed.

In general, I expect we'll see much more virtualization in this area in the future. For example, today Cisco's Application Extension Platform (AXP) products are physical x86 cards you stick into a router to run server workloads. It would be plain silly not to take advantage of the well-known consolidation benefits of virtualization to accomplish the same thing. (That's pure speculation, but as I said... silly.)

package management and embedded Linux distributions

A while back, when it looked like the Kuro Box would actually go somewhere, I bought the original model. Among other things, this consists of a 200MHz Freescale e300 core (similar to PowerPC 603e) and 64MB of RAM. No serial port even, but through u-boot it has netconsole and netboot support. I had a decrepit version of Debian installed, and Debian's package management tools frustrate me to the point that I was helpless to get the thing upgraded (something about packages mysteriously "being held back").

For a system like this, the real value of a Linux distribution is the frequency of its security updates. There are some embedded distributions I could have messed with, but I have no idea how reliable their updates are, and I really don't want to be responsible for that myself. I started looking for a "normal" Linux distribution to install. (Unfortunately, the number of mainstream Linux distributions with PowerPC support are shrinking...)

I'm really a Fedora guy. Back in university I did some packaging for LinuxPPC and Yellow Dog Linux (both of which were Red Hat variants), and I'm comfortable enough with RPM to bootstrap a system from just about nothing. So I tried the Fedora installer, and when anaconda failed miserably with netconsole, I painfully installed it package by package.

Long story short, yum (the Fedora software installer) requires an absurd amount of RAM... way more than the 64MB my little Kuro has. The simplest yum operation was taking hours, and I could see I was well into swap. So after all that work, the box was still useless. I ran out of play time, unplugged the thing (since I couldn't install any security updates for it), and there it has sat for another 6 months.

Today I ran across a blog post comparing the speed and memory consumption of yum and zypper (OpenSUSE's package software installer). I don't know much about OpenSUSE, but I will be trying it next...

leveraging Linux for virtualization: the dark side

I work on KVM, which is a relatively small kernel module that transforms the Linux kernel into a hypervisor. A hypervisor really is a kernel: it contains a scheduler (at least the good ones do ;), device drivers (at least interrupt controller, probably console, maybe more), memory management, interrupt handlers, bootstrap code, etc.

This is the key observation behind KVM's design. "Hmm, we need a kernel... and hey, we've already got one!" We just need to add some code to make it schedule kernels instead of userspace tasks. In fact, one of the major technical faults of the Xen project was that it needed to duplicate — often copy outright — Linux code, for features such as power management, NUMA support, an ACPI interpreter, PIC drivers, etc. By integrating with Linux, KVM gets all that for free.

There is a drawback to leveraging Linux though.

pro	con
use Linux's scheduler	stuck with Linux's scheduler
use Linux's large page support	stuck with Linux's large page support
get lots of fancy Linux features	stuck with the footprint of Linux's fancy features

Seeing a theme here? Let me share a little anecdote:

My team had been doing early development on KVM for PowerPC 440, and we were scheduled to do a demo at the Power.org Developer's Conference back in 2007. Unfortunately we weren't able to get Linux booting as a guest in time, but we had a simple standalone application we used instead. So when I say "early development" I mean "barely working."

A friend of mine walked up to the demo station and asked "Does nice work?" Now remember, basic functionality was missing. We couldn't even boot Linux. The only IO was a serial console. We had never touched a line of scheduler code, and certainly hadn't tested scheduling priorities. Despite all that, nice just worked because we were leveraging the Linux scheduler.

There's a down-side though. The Linux scheduler is famously tricky, and almost nobody wants to touch it because even slight tweaks can cause disastrous regressions for other workloads. The Linux scheduler does not support gang scheduling, where all threads of a particular task must be scheduled at once (or not at all).

Gang scheduling is very interesting for SMP guests using spinlocks. One virtual CPU could take a spinlock and then be de-scheduled by the host. Unaware of this important information, all the other virtual CPUs could spin waiting for the lock to be released, resulting in a lot of wasted CPU time. Gang scheduling is one way to avoid this problem by scheduling all virtual CPUs at once.

Since Linux doesn't support gang scheduling, and only a handful of people in the world have the technical skill and reputation to change that, that's basically a closed door.

This is just one example, but I think you can see that re-purposing Linux for virtualization is a tradeoff between functionality and control. If one were to write a new scheduler for a hypervisor, they'd need to implement nice themselves... but they would also be free to implement gang scheduling.

Wind River Linux 3.0 adds KVM

Wind River recently released Wind River Linux 3.0, including KVM support (on x86 systems of course).

Wind River is better known for their VxWorks embedded RTOS, which traditionally has been one of the dominant operating systems in the embedded industry, and still is today. After criticizing Linux and the GPL (as VxWorks competition) for years, in 2003 the company gave in and started moving towards Linux support, including its own Linux distribution. Today Wind River Linux is appearing in more and more places in the embedded Linux market. I think it's considered #2 after MontaVista, though I admit I don't know the relative market shares there.

In some ways, KVM support in Wind River Linux isn't a big surprise, because we already know that Wind River believes in embedded virtualization so much they're writing their own hypervisor.

In other ways, it is a surprise, because KVM is a hypervisor too, and as such might compete with their own hypervisor. I suppose they will have lots of internal conversations about market positioning and how to convince the world they're not competing with themselves, but I guess every sufficiently large company has the exact same issue.

Anyways, the one big takeaway from all this is that Wind River seems to be saying that KVM is good enough for embedded systems. Since I've been saying the same thing for a while to a sometimes-skeptical audience, I'll take it. ;)

design tradeoffs in hardware virtualization

I mentioned that the Power ISA version 2.06 was published recently, which added a model for hardware virtualization on "Book E" embedded processors. (The model for hardware virtualization in server processors, such as POWER4, has been around for years.)

The only reason to add hardware support at all (for anything, not just virtualization) is to improve performance. You can do pretty much anything in software; it's just faster to do it in hardware. For example, people have run emulators and JVMs for years, and that gives you a virtual machine without hardware support. We've even demonstrated virtualization without hardware support with KVM on PowerPC 440.

So the goal for virtualization hardware support, to allow the guest kernel to access as much hardware state as possible without host intervention. In an ideal world we could just duplicate all processor state and allow the guest free access to its own copy... but hardware costs power, heat, and die space.

As a compromise, the Book E virtualization architecture duplicates only the state accessed in the fast path of normal guest operation. So there are guest-writable copies of registers like SRR0-1, SPRG0-3, ESR, and DEAR, which are heavily used by the guest in its interrupt vectors. However, registers which are only used for hardware initialization are not duplicated: when the guest tries to access these registers, a privilege fault occurs and the host/hypervisor emulates the instruction. Slow, but (hopefully) only for operations that don't need to be fast. Similarly, some interrupt vectors (such as alignment interrupts) are only delivered to the host, and at that point it is software's responsibility to implement interrupts delivery to the guest.

In contrast, the virtualization architecture for x86 doesn't duplicate register state, but rather provides instructions for atomically transferring a boatload of register state to and from memory. This definitely does not fit with the RISCish philosophy of the Power architecture, where memory accesses are performed only by load and store instructions. I'm not a hardware person, but I can guess that implementing the x86 technique is rather difficult... and I guess that's the whole point of CISC and RISC. :) I can say that I really appreciate the flexibility when hardware provides simple tools and software can use them how it likes.

Anyways, I can't judge one way as better than the other because I don't understand the hardware implications, but that's really the point I'm trying to make: implementing functionality like this is all about design tradeoffs between hardware and software.