Comment by pseudohadamard
2 months ago
RISC-V truly is the RyanAir of processors: Oh, you want FP maths? That's an optional extra, did you check that when you booked? And was that single or double-precision, all optional extras at an extra charge. Atomic instructions, that's an extra too, have your credit card details handy. Multiply and divide? Yeah, extras. Now, let me tell you about our high-end customer options, packed SIMD and user-level interrupts, only for business class users. And then there's our first-class benefits, hypervisor extensions for big spenders, and even more, all optional extras.
So it's modular. This is normally considered a good thing. It means you don't have to pay for features you don't need.
The ISA is open so there's no greedy corporation trying to upsell you. I mean there's an implementation and die area cost for each extension but it's not being set at an artificial level by a monopolist.
There's a good chance you're actually paying more for the features you don't need. Preparing an EUV mask set costs something like 30 million dollars (that figure may be out of date, i.e. it could be more now). So instead of a single mask set with everything on the device, whether you need it or not, you're paying $30 million for each special-snowflake variant. This is why vendors do a one-size-fits-all version of many of their products and then disable the extra functionality for the cheaper market segments, because it's much, much cheaper than making separate reduced-functionality devices.
It's a good thing in many cases but not if you're going to be running applications distributed as binaries. Maybe if we go the Gentoo route of everybody always recompiling everything for their own system?
Then you stick to RVA23, which is comparable to ARMv9 and x86-64v4.
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But that means a port of Linux can’t be to RISC-V, it has to be to a specific implementation of RISC-V, or if sufficient (which seems still debatable) to a specific common RISC-V profile.
>which seems still debatable
In what way are RISC-V profiles debatable? Canonical is spearheading the RVA23-as-a-default movement and so far, it seems that there are no heavy objections towards that effort (beyond the usual "Canonical sucks" shtick that you see in every discussion involving Canonical)
You can target the minimum instruction set and it'll run everywhere. Albeit very slowly. Perhaps you use a fat binary to get reasonable performance in most cases.
This isn't easy but it can be done (and it is being done on x86, despite constantly evolving variations of AVX).
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I don't agree with that comparison.
RyanAir is about exploiting consumers, with bait-and-switch and shitty terms and conditions.
RISC-V's modularity is about giving choice to hardware designers, so they can pick and choose just those features that their solution needs, and even allow for custom extensions.
RISC-V's modularity is for academia. 1) for education, where students learn/use/work on simple processors, 2) for research in new types of hardware and extensions, where ease of implementation or ease of creating a custom extension is important.
Extensiosn are not just for academia. If I am building a microcontroller to control the storage media I am selling (eg. hard drives), why do I need to implement a bunch of features I am not going to use? What about my flow rate monitor? Or my pacemaker?
In some of these, less silicon means less power means more better. Like that last example.
Then x86_64 is the cable television service of processors. "Oh, you want channel 5? Then you have to buy this bundle with 40 other channels you will never watch, including 7 channels in languages you do not speak."
>Multiply and divide
And where it actually mattered they did not introduce a separate extension. Integer division is significantly more complex than multiplication, so it may make sense for low-end microcontrollers to implement in hardware only the latter.
There is Zmmul for multiplication-but-not-divide.
RyanAir is the least expensive right? And it still gets you there?
I would be ok with that if it was a valid analogy.
It is valid in microcontroller land. There, the chip and the software are provided by the same party. So you can select for exactly the RISC-V features you need and save yourself some silicon. That sounds like a win to me.
At the application level, like a server or a desktop, that would be a disaster because I get my hardware and software from different people. How do the software guys know what hardware to target? Well, that is exacly why RVA23 exists.
What does RVA23 mean? It is the RISC-V "Application" profile. It allows you to build software to a single hardware target and trust that hardware makers will target the same proifle. RVA23 is like saying x86-64v4. Both are simple names for a long list of extensions (flags) and assumptions that you expect the hardware to honour. So, when Ubuntu 26.04 says it requires RVA23, it means that all the software built on it can assume those features. No a la carte.
The reason RVA23 is geting so much attention is that it has essentially the same feature set as modern ARM64 or x86-64. Software will be able to target this profile for a long time. There may be a new profile in a few years time, like RVA30, but hardware that implements that will still run RVA23 software (just as x86-64v4 hardware will run x86-64v1 software). Hardware built for profiles before RVA23 may be missing features modern applications expect.
I guess you could say that RVA23 is British Airways Business Class.
If you really want to support hardware designed before RVA23, almost everything you would want to run pre-built software on supports RVA20. And again, your RVA20 stuff will run fine on RVA23 hardware (but with fewer features--like no vectors). So maybe no in-flight meal, but it will get you there.
Yes, adding instructions to your ISA has a cost