[PATCH] riscv: Work to remove kernel dependence on the M-extension

[Michael T. Kloos]

Some other thoughts:
It sounds like I am not the first person to want this feature and I
probably won't be the last.  I created the change for my own reasons, the
same as any other contributor.  I think we all know that I can not pull
out some chart and say, "This many people want this and here is why."  I
live in central Ohio and have been doing this as a hobby.  I don't even
know anyone else who knows about systems and operating system development.
If the justification that you are looking for is that I as some
hypothetical developer at a major tech company is about to release a new
RISC-V chip without M support but we want it to run Linux, I can not
provide that answer.  It sounds a bit like some software or hardware,
chicken or the egg anyway.  Trying to maintain my own fork if people
start contributing patches with incompatible assembly scares me.

	Michael

On 3/10/2022 4:31 AM, Michael T. Kloos wrote:

> As far as I can tell, it's simply a compiler flag to enable or disable it.
> The only complicating issue that I found was the BPF JIT.  That is a
> special case amounting to a compiler within the kernel itself and can be
> patched or set to depend on M.  There is still the BPF interpreter.  The
> kernel doesn't depend on the C-extension.  I have grepped through the
> source code where that symbol is referenced, the checking of instruction
> alignment constraints, and various checks used to make that work are much
> more extensive than I expected, yet that is supported.  To quote the
> commentary from the beginning of chapter 2 (Base-I) of the RISC-V spec:
>> "RV32I was designed to be sufficient to form a compiler target and to
>> support modern operating system environments. The ISA was also designed
>> to reduce the hardware required in a minimal implementation. RV32I
>> contains 40 unique instructions, though a simple implementation might
>> cover the ECALL/EBREAK instructions with a single SYSTEM hardware
>> instruction that always traps and might be able to implement the FENCE
>> instruction as a NOP, reducing base instruction count to 38 total.
>> RV32I can emulate almost any other ISA extension (except the A extension,
>> which requires additional hardware support for atomicity).
>> In practice, a hardware implementation including the machine-mode
>> privileged architecture will also require the 6 CSR instructions.
>> Subsets of the base integer ISA might be useful for pedagogical purposes,
>> but the base has been defined such that there should be little incentive
>> to subset a real hardware implementation beyond omitting support for
>> misaligned memory accesses and treating all SYSTEM instructions
>> as a single trap."
> This is all a noble goal of RISC-V, to create a minimal, yet robust ISA.
> I'm not arguing to support no-A, as stated in the commentary, atomic can
> not be emulated (I mean, maybe you could by taking over sscratch) and
> the atomic properties of the CSR instructions.  But that does seem hacky.)
> and everything else is handled in the Base-I (Not including Zicsr).  The
> kernel supports rv(32/64)ima so to draw the line in the sand that
> rv(32/64)ia is unacceptable seems silly.
> One of my concerns here is that if support is not added, that people may
> start writing assembly that uses M instructions and other non-portable
> stuff.  This will make a future port more difficult.
> I do not have real hardware like this but I do have an emulator that
> implements rv32iasu.  Eventually I want to build a HART implementation
> out of real hardware logic chips.  Adding M, adds hardware complexity and
> I think that it is better to do in software for such a minimal case.
> Sure it will be slower than having native hardware M support.  Firmware
> emulation will be even slower than that.  I'd much rather have the option
> in the kernel to switch this on or off.
> I am not trying to argue that the kernel should bend over to support my
> pet projects, but it seems quite strange and arbitrary to draw the line
> here over such a beautifully modular and minimal ISA.
>> there's an infinite amount of
>> allowable RISC-V implementations, we'll all go crazy chasing them
>> around.
> 2 points on that:
> 1) Then it would seem logical to have the kernel target the most minimal
> implementation to get the widest support.
> 2) I actually don't think this is entirely true, or at least not in the
> sense that it is relevant to the kernel.  Atomics are needed for a modern
> system.  The kernel needs to know about the floating-point support because
> it needs to handle the clean-up and context-switching of the additional
> floating point registers.  The kernel needs to know about the C extension
> because of the ramifications to alignment.  Everything else is just gravy.
> If sometime in the future the L-extension or the B-extension are finished
> and implemented in future hardware, unless they make some very strange
> decisions, I don't see why a kernel built today couldn't run on that
> hardware with userspace software taking advantage of those native features.
> Sure, in that situation, the kernel binary wouldn't be taking advantage of
> them.  But then wouldn't you want to add CONFIG_RISCV_ISA_L and
> CONFIG_RISCV_ISA_B in said future kernel so that the compiler is passed an
> -march value such that the kernel can take advantage of these new native
> hardware features?
> It seems to me that this is the grand vision in the modularity of both the
> RISC-V ISA and the C programming language.  We aren't writing everything in
> assembly for a reason.  I don't think it makes sense to stop one extension
> away from what the RISC-V spec documents themselves state is the intended
> minimal system capable of running a full modern operating system.  I think
> it is an arbitrary limit to the portability of the kernel.
>      Michael
> On 3/10/2022 2:34 AM, Palmer Dabbelt wrote:
>> On Wed, 09 Mar 2022 23:06:22 PST (-0800), Christoph Hellwig wrote:
>>> Why?
>> I have no idea, but this has come up a few times before.
>> IIRC the original port had a no-M flavor (don't remember if it even made
>> it to the original patch submission, but it was around for a bit).  We
>> decided to drop this under the theory that nobody would use it:
>> essentially, if you can afford the handful of MiB of memory required to
>> run Linux then you've probably got a multiplier.
>> If someone has hardware that lacks M and users care about running Linux
>> on that then I'm happy to support it.  I'll still point out the
>> silliness of that decision, but if it's too late to change things then
>> I'd rather support the hardware.  If it's one of these "fill out every
>> possible allowed ISA flavor, even if nobody has one that runs Linux"
>> then I don't see a reason to bother -- there's an infinite amount of
>> allowable RISC-V implementations, we'll all go crazy chasing them
>> around.
>> FWIW: to a first order, that applies to the no-A stuff as well (though
>> that got dropped because the Arm folks pointed out a way to support that
>> was way better than ours).
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