Re: [PATCH v1 2/2] riscv: Clean up module relocations

[Luke Nelson <lukenels@cs.washington.edu>]

Thanks for the patch!

> Also RISC-V has a number of instruction pairs to
> generate 32bit immediates or jump/call offsets. Eg.:
>
> lui   rd, hi20
> addi  rd, rd, lo12

On RV64, both hi20 from lui and lo12 from addi are sign-extended to 64 bits.
This means that there are some 32-bit signed offsets (in the range
[2^31-2^11, 2^31-1])
that are not encodable using (lui+addi), (auipc+jalr), etc. (see
discussion at [1]).

The following note is from the ISA manual:
>>> Note that the set of address offsets that can be formed by pairing LUI with LD,
>>> AUIPC with JALR, etc. in RV64I is [−2^31−2^11, 2^31−2^11−1].

The existing code and the new code both seem buggy if the offset happens to
be a 32-bit int but falls outside of the encodable range.

> +       if (offset != (s32)offset) {
> [...]
> +       if (offset != (s32)offset) {
> [...]

These checks should probably be replaced with something similar to
what's used in the RV64 BPF JIT here: [2],
except that this code should check if using RV32 or RV64, since the
encodable range differs for each.

> My hope is that we can eventually factor out the code to generate
> immediates and instructions so it can be reused both here, in the
> jump-label code and in the bpf-jit code, but let's take it
> one step at a time.

This sounds great! Having fewer copies of RISC-V encoding logic around will
hopefully decrease the likelihood of bugs :) Some other archs already
have shared
infrastructure for doing instruction encoding (e.g., in
arch/arm64/kernel/insn.c);
we should consider doing something similar for RISC-V.

- Luke Nelson

[1]: https://groups.google.com/a/groups.riscv.org/forum/#!topic/isa-dev/bwWFhBnnZFQ
[2]: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=489553dd13a88d8a882db10622ba8b9b58582ce4

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