How retiring segmentation in AMD64 long mode broke VMware (2006)

http://www.pagetable.com/?p=25

by userbinator • 12 years ago

53 11 12 years ago

Great share!

I think this makes for an interesting cautionary tale: when removing features, no matter how little-used they may appear to be, there's always the possibility that it will break something very important and useful that depends on them. The path to the solution also shows how this attempt to remove complexity actually resulted in even more complexity later on, in the form of diverging virtualisation extensions and non-uniform support for segment limits.

Segmentation isn't the only one, however; many of the existing seldom-used one-byte instructions also inexplicably went missing (became completely invalid - they didn't reuse them for prefixes or anything like that), and among them were SAHF/LAHF which also turned out to be important for virtualisation. In this case the solution was that both AMD and Intel put them back sometime later, and had to add an extra "feature bit" to indicate this. It's quite absurd considering that these instructions were present in 32-bit mode, and leaving them unchanged in 64-bit mode from the beginning would've avoided this issue completely.

As successful as x86-64 is, I definitely think the 64-bit transition could've been much better, similar to how the 32-bit extensions that came with the 386 fitted nicely into the existing instruction set and could even be used from 16-bit mode. In contrast, there is no way to use 64-bit registers in 32-bit code despite there being reservations in the existing instruction set that would've made it possible. V86-mode, which has been present since the 386, could've been extended in a relatively straightforward manner to make virtualisation easier.

nnx• 12 years ago

In contrast, there is no way to use 64-bit registers in 32-bit code despite there being reservations in the existing instruction set that would've made it possible.

Isn't it what the X32 ABI do?

https://en.wikipedia.org/wiki/X32_ABI

ANTSANTS• 12 years ago

No, x32 is still long mode (64-bit), it's just an ABI that restricts all memory allocations to 4 gigabytes of address space so that 4-byte pointers can safely be used instead of 8-byte ones. You can get a similar effect by manually mmaping memory into the lower parts of the address space even in a regular 64-bit program (luajit relies on this), but you can't stop malloc, the kernel, other libraries, etc. from using the rest of the address space.

On x86 CPUs since the the 386, you don't need a hack to make 32-bit mode look like a 16-bit mode, you literally can access the full 32-bit registers in 16-bit modes with an operand size prefix.

rbanffy• 12 years ago

Backwards compatibility has to be used with care: too much of it and your new product will never have software of its own and will only be a newer version of your old product. The Commodore 128 is a good example of this.

yuhong• 12 years ago

Windows x64 used to emulate SAHF/LAHF and 3DNow! PREFETCH. This was eliminated in x64 Win8.1, ending support for all 90nm Prescott etc CPUs (the newer 65nm Cedar Mill etc are still supported).

metacorrector• 12 years ago

yeah but... the real problem lies elsewhere...

VMware (and that style of virtualization) is so popular because of the utter failure of OSes to do what they are supposed to do.

Protected multiuser multitasking operating systems are supposed to be able to protect processes from impinging on one another, and they can't do it. The result is that on a very large scale we have datacenters that spawn VMware virtual machine after VMware virtual machine, a "make everything look like a nail so we can use this hammer" approach that is wasting huge amounts of resources.

There would still be a use for VMware if OSes worked, but nothing like we see today. People should get back to work on fixing OSes so a cloud host could actually run processes from different customers in a lightweight way and have them not screw each other up.

Karellen2• 12 years ago

"Protected multiuser multitasking operating systems are supposed to be able to protect processes from impinging on one another, and they can't do it."

Can you elaborate? In what way are they failing here?

modeless• 12 years ago

Ideally, I should be able to download an untrusted binary application from the internet, run it on my local machine, and be able to trust that it won't be able to steal my passwords, read my contacts and pictures and documents, use my camera and microphone, drain my battery, use up all my CPU and RAM and network and disk resources, freeze my system, or do any other nefarious thing without my permission. OSes have failed so misererably at this that web browsers have had to pick up the slack and the web is slowly morphing into a bad operating system.

lmm• 12 years ago

Say you have three unreliable programs that occasionally leak memory, spin CPU etc. (yes, it would be nice if all our programs were perfect, but they're not). It should be possible to run these three programs on the same server in such a way that the failure of one won't affect the others (so e.g. you have 3 servers, each runs one of each program, and you load balance between them, and you have some system that eventually detects when one of the instances fails. So as far as the outside world is concerned, all your programs are running reliably). At the moment, the most practical way to do this is to run 3 different VMs on each server, one for each program. Which is insane. There are some encouraging recent developments (e.g. docker/lxc), but it should be easy to do that kind of isolation purely at the OS level.

How retiring segmentation in AMD64 long mode broke VMware (2006)

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