What does "Hot Patchability" mean and what is it for?

I got a question on my earlier ABI post about Hot Patchability, so I thought I'd
go into excruciating detail on that one, since it's not quite as complicated as
exception handling

What the heck does Hot Patchable mean?

Hot patchable means that, primarily, you're able to take a running application and,
given sufficient privileges, atomically changes all calls from function A to switch
to function B. At a high level, this allows you to patch a running process without
requiring that the process be stopped [2 steps better than a reboot!] This is really
important in the "Server Up-Time" scenarios that are becoming more and more common.
It allows a patch to be deployed without even stopping a process [for more than
the length of a standard context switch, anyway].

Why should I care?

Maybe you won't, but if you're deploying server code, you probably don't want to
have to terminate the process to deploy a security patch, right? Microsoft's customers
really don't like it when we make 'em restart their server processes.

Okay, I care, how does it work?

First, remember that I'm a compiler guy, not a debugger guy, or a kernel guy, or
anything else, but here's my understanding: First, you build your patched function.
This is a non-trivial amount of work. It must have a compatible function
signature with the function it's replacing. It also needs to have special code to
access any globals that are needed, since it's generally loaded as a separate DLL
[though I imagine a tricky debugger guy could do some kind of nutty code injection
where that isn't necessary]. Once you have the code authored, you have to deploy
the patch. This is where the ABI restrictions come in.

First, all functions must start with a 2 byte (or larger) instruction - if you start
with a push, stick a size prefix on it. Second, all functions must be preceded by
6 bytes of padding. Finally, any image that needs to be hotpatchable should also
have some amount of 'scratch space' within 2GB of it's image location. Why, you
ask? Well, here's how hot-patching actually works:

Pause the process and load your hot patch dll into the address space [again, I don't
know all the mechanics for this, but I know it's not too difficult. Next,
write to that 'scratch space' the address of your hot patch function. Now, write
the 6 bytes JMP [PC-relative scratch space] into the 6 bytes of padding
before the function you're trying to replace. Finally, write the 2 bytes jmp PC-6
into the first two bytes of your function. Resume the process, and your hot patch
function is merrily running instead of the old one.

How does that work again?

The point of the 2 byte instruction at the start of each function is so that you
don't ever have to worry about pausing your process in the middle of the two bytes
you're going to change with the jmp PC-6 instruction. Nothing else
is really interesting. You setup a launch pad to your scratch space, which is where
the target address lives. No rocket science, here, nosiree.

What if I don't want hot-patchability

Honestly, I don't think there's anything that prevents you from breaking these particular
rules, except the cost is so minimal, there's really just no good reason not to
do it. X86 has something like a 30% hot-patchable kernel. And x64 has a 100% hot-patchable
kernel. You tell me which one is better.