[clug] Why virtual x86 machines?

Peter Saville savillep at protonmail.com
Thu Aug 20 12:49:13 UTC 2020

nice post, enjoyed it. the thought of un-boxing, wiring, networking and troubleshooting $14,000 worth a rPi's. that's a hard no from me...

Not exactly sure about the CMS question, and correct me if I'm wrong, but KVM is built into the linux kernel so it might look like a type 2 hypervisor, but is a native hypervisor that talks directly to hardware.



On 20/8/20 10:05 pm, Steve McInerney via linux wrote:

> What's the max cores/ram on a pi?
> Can I run multiple VMs with 4, 8, 24 cpus and 8/16/32/64/128 G ram?
> 10GiB nic for disk IO?
> Separate nics for other traffic?
> Do pi's come in nice neat rack units to make racking easy in a DC? Dual
> power etc etc.
> If I need local storage, do they come with built in raid?
> What about OOB/LOM/IDRAC et al?
> etc etc
> I don't know much about pi's beyond a quick google - hence the
> questions, but that's the sort of things I'm looking for in large and
> small cloud usage hardware.
> If Pi's can do all the above, then sure, they're a useful alternate,
> please point me at some built hardware and costings!
> Cheers!
> Steve
> On 2020-08-20 21:09, Hugh Fisher via linux wrote:
>> Inspired by the questions about KVM, I've been doing some reading on
>> virtual machines and containers and some of the other new abstraction
>> & protection mechanisms being used today. I like to write things down
>> to clarify my thinking, and am posting this to the list in the hope
>> that people with more knowledge will correct me if I'm wrong. And I do
>> have questions, at the end.
>> First up I'm not including the Java Virtual Machine, or the similar
>> bytecode like systems used in .NET, Python, etc. Those are designed
>> for user level programs, not OS kernels. And I'm not including
>> emulation/simulation where machine instructions are interpreted by
>> another program, because then it's turtles all the way down. A 6502
>> Apple II running ProDOS can be emulated by a program on a M68030
>> Macintosh running System 7 which itself is being emulated by a program
>> running on a PowerPC Macintosh running MacOS X ...
>> So, a virtual machine, usually associated with a hypervisor and guest
>> operating system kernels, executes as many as possible machine
>> instructions on the actual CPU hardware. (Using the old definition
>> that you can kick hardware, but only swear at software. And just skip
>> over microcode.)
>> From my old Andy Tanenbaum textbook the first virtual machine in
>> widespread use was VM/370 for IBM mainframes, around 1970. I think the
>> history is important because of a question I'll bring up later.
>> A 370 series IBM mainframe, ancestor of the backwardly compatible zOS
>> mainframes still sold today, could easily cost a million dollars. A
>> 370 mainframe would run an entire bank financial system, or an entire
>> airline reservation network. Which was awkward if a new release of the
>> operating system was due and you wanted to test that all your software
>> would still work. Shut down everything while you reboot into a beta
>> OS? Buy another million dollar mainframe just for testing?
>> VM/370 was what today we call a hypervisor, that could run multiple
>> guest operating systems side by side on a single CPU, providing each
>> operating system its own "virtual 370". Now the bank could run VM/370
>> on its single mainframe, with say 90% of machine resources allocated
>> to the guest production OS and the rest given to whatever the
>> developers wanted.
>> This was a major technical achievement. Then, like now, the operating
>> system distinguished 'user mode' from 'kernel' or 'privileged' or
>> 'system' mode. User mode machine instructions could not modify virtual
>> memory page tables, issue DMA instructions to IO hardware, and so on.
>> Only kernel code could do that. So unlike a regular operating system
>> the hypervisor had to work with guest operating system kernels
>> executing privileged machine instructions. The guest kernels didn't
>> know that they were running on a virtual 370, so it was up to the
>> hypervisor to ensure that if, say, one guest OS disabled interrupts,
>> this wouldn't shut down every other guest.
>> Once IBM got VM/370 to work, it was a big hit. It was so popular both
>> inside and outside IBM that some new instructions and microcode
>> modifications were added to the 370 machine architecture to make IO
>> and memory paging within the guest operating systems more efficient.
>> And IBM then developed CMS, a hypervisor-aware operating system kernel
>> designed to run only on VM/370. A conventional OS protects multiple
>> users from affecting each other, whether deliberate or accidental. CMS
>> was a single user OS, and VM/370 gave every user their own copy on
>> their own virtual 370. Even if there was a kernel exploit in the CMS
>> operating system (not the hypervisor), the only person you could
>> attack would be yourself. CMS was a smaller and simpler operating
>> system because it didn't duplicate functions that VM/370 was already
>> doing.
>> Now fast forward to the 21st century. If you
>>     cat /proc/cpuinfo
>> on an x86 Linux system and you see 'vmx' in the output, you have the
>> Intel virtual machine hardware extensions. The original x86
>> architecture had Ring 0 for privileged machine instructions as used by
>> operating system kernels. The virtual hardware extensions add Ring -1
>> for a hypervisor such as VMWare, which can run multiple guest Linux or
>> MS Win kernels side by side. Each of these thinks it is running with
>> Ring 0 privilege and can update page tables, issue IO instructions to
>> PCI slots or disk controllers, and so on.
>> So Intel virtual x86 is just like VM/370. Except ... x86 computers
>> don't cost a million dollars.
>> So my most important question, why bother? Just buy another CPU.
>> I did a quick price comparison on
>> www.mwave.com.au
>> . The cheapest Intel
>> Xeon is about $4,000 and it's possible to spend $14,000 if you want
>> to. For those amounts of money you could buy a shoebox up to shipping
>> container full of Raspberry Pis, complete 64 bit Ghz systems with RAM
>> and ports. Or if you have to stay within the x86 family, Intel
>> Celerons are at least five times cheaper than Xeons. Looking instead
>> at power budget, the cheapest Xeon CPU consumes as many watts as five
>> entire Raspberry Pis.
>> Looking at these prices I understand why Intel want us to virtualise
>> x86 CPUs and run multiple guest operating systems. I don't see why
>> anyone else would want to.
>> But since datacentres and cloud systems do use hypervisors I must be
>> missing something. Anyone want to explain?
>> Second question, are there custom Linux kernels designed to run on
>> hypervisors? Not a Container OS, which I think is something else, but
>> like CMS designed to be single user or otherwise not duplicate what
>> the hypervisor is already doing?
>> And lastly I'm assuming that there's nothing in virtual x86 design and
>> implementation that VM/370 didn't already do. Am I wrong? What new and
>> interesting uses for hypervisors have been thought of?
>> --
>>         cheers,
>>         Hugh Fisher
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