Comment by Koshkin | Hacker News Reader

Comment by Koshkin | Hacker News Reader

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But in all fairness, why would anyone just "want a non-x86 desktop" (as in: "anything but x86"). Doesn't make sense to me...

It can be useful to test code that's supposed to be portable on a system with a weaker than x86 memory model.

And a self-synchronizing instruction stream can be a security advantage and a fixed instruction width means Power and AArch64 are trivially self-synchronizing. And really just not having the dominant ISA is a pretty big security advantage in some cases.

I'm pretty sure that neither of those is a huge deal for most people but I can see wanting it.

ELI5: How, specifically, does AArch64 have a "weaker than x86" memory model?

Googling those terms will turn up lots of hits, e.g., https://preshing.com/20120930/weak-vs-strong-memory-models/

When multiple cores access the same memory location, it is expensive for one core to invalidate the cache of another or ensure operations don't get re-ordered in either core. These days most architectures when reading or writing shared data require memory barrier instructions to guarantee your core to sees writes from another, or other cores to see your writes, in a timely sequential fashion like we expect when writing code that accesses variables.

Historically there were architectures that would reorder these accesses in a "lax" way, making very few guarantees about what you will see from another core, on the theory that it will cost less to keep things synchronized between cores (most data is not shared anyway, so why waste work trying to create a unified view across cores? The CPU can also reorder work for better efficiency.). Intel is historically one of the most conservative, strict-ordering architectures, requiring fewer barrier instructions and creating the illusion that reads and writes more or less occur on a single timeline.

See also:

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

> And a self-synchronizing instruction stream can be a security advantage and a fixed instruction width means Power and AArch64 are trivially self-synchronizing.

AArch64 is aligned, but not self-synchronizing. While it would not be possible to execute, reading code at an arbitrary offset can still create be valid code. I'd imagine Power has the same issue, since it's really hard to do this generally if you'd like to have a sane encoding and support immediate.

Somewhat interestingly, x86 is variable length but I have heard that it is often "eventually self-synchronizing": apparently if you start it off at the wrong offset, it will decode a couple of instructions incorrectly but usually end up disassembling to the correct ones.

Generally fixed width instruction sets require that instructions be aligned, that is instruction addresses end with two 0s if they're 32 bits. One benefit of this is that you can make a jump 4 times as long for a given constant size. In the case of AArch64 this lets them do 128MB branches with a 26 bit signed constant or 1 MB conditional branches with a 19 bit constant.

Another benefit is that the fetch stage doesn't have to handle corner cases like the instruction crossing cache line boundaries. I don't think the security implications were anything the designers cared about but they're a third benefit. Oh, and I think some language designers have stored garbage collection related information in the least significant bits of stored addresses since it doesn't affect flow control but I wouldn't swear to that.

You're right that a natural x86 instruction stream will tend to synchronize itself fairly quickly. The problem is a malicious instruction stream that can be designed not to do that for at least long enough to do its thing.

I don’t know why you’re getting downvoted. It’s a fair question to ask.

“Why can’t I buy” is essentially equivalent to “why doesn’t anyone sell”, which is basically “why isn’t there a market for?”

It’s possible to have a conversation around “why isn’t there a market for ARM desktops?” or equivalent but a conversation about “why isn’t there a market for anything but x86” would basically be enumerating the alternatives and examining each.

Nobody is going to open a store aimed at the “anyone but x86” market.

It's not that crazy. In other fields, there are stores for "alternative ___". There's a record store a couple blocks from me which sells every physical medium except CD. There's a car dealer which sells cars for every fuel except gasoline. There's many restaurants which serve foods with every ingredient except meat. There's government departments, education departments, and stores which deal in every language except English.

When one way is dominant, it's not uncommon for all the others to get rolled up into an "alternative" grouping. Any one of them by itself would be too small to amount to even a small store, but all of them together are a decent set.

“Why can’t I buy?” ≠ •Why doesn’t anyone sell?” ≠ “Why isn’t there a market for?”, not even close.

Why can’t I buy talent (in the sense of myself being talented at something)?

Because nobody sells it? There’s no market for it? Nah.

I’m an economist but the notion of everything being tradable on a market needs to be curtailed because it simply isn’t true.

x86 may be locally optimal, but I don't trust it not to be a dead end, or at least, have its lunch eaten by something else; I personally am interested in non-standard configurations just to make sure that we can try new things.

You want better performance per watt. Apple sells millions of iPads and Google OEMs sell some number of ARM Chromebooks and Tablets. Amazon is selling little android tablets as low as $40.

The desktop is essentially a legacy model as it stands today. Nobody is offering ARM because Windows doesn’t support it, and there really aren’t other options. Even enterprises are shipping the cheapest crap possible for PCs and innovating on mobile.

You’ll see some change when Apple ships ultralight laptops that aren’t as compromised as the intel platform devices.

Maybe people just want to play heavily modded Minecraft on a Raspberry Pi that they can quickly set up and shotgun to eager nephews all across the country. Just throwing that out there.

If your work is all non-x86 then having powerful non-x86 box handy helps a lot.

maybe you detest x86 and love aarch64. it's like cars. i dunno...why do you need to understand it for it to be a "thing" that exists in the world outside of your head?

it's the 64bit leap from the acorn risc machine, for crying out loud, it's its own goal, wantable for it's own sake. pity that riscos will never be 64bit lol

Precisely: I want to do it because it’ll be interesting, the same reason I had a BeBox in the mid nineties and a Plan9 network in the 2000s.

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