fix(riscv): set FS before fp restore/clear in switch_to

[LetsWalkInLine]

• Resubmitted to main branch
• Supersedes: fix(riscv): set FP state before restore or clear #28

Background

When booting downstream StarryOS on riscv64 with RustSBI, the kernel could panic with:

• Unhandled trap Exception(IllegalInstruction)
• stval=0xf2000053
• stack trace around task switching / fp-simd context switch path
  The same workload could boot under other firmware setups, indicating a firmware-dependent FP init assumption.
  (Downstream context: [bug] 使用RustSBI启动StarryOS失败 Starry-OS/StarryOS#130, fix(riscv64): boot StarryOS with RustSBI on riscv64 without IllegalInstruction trap Starry-OS/StarryOS#131)

Root Cause

In axcpu RISC-V fp-simd context switching, FP instructions can be executed in restore/clear paths while sstatus.FS is still Off on some boot paths/harts.
When FS=Off, executing FP instructions (e.g. fld / fmv.d.x) traps as IllegalInstruction.
So the issue is not OS entry order itself, but a missing local precondition in the axcpu FP context switch path.

A common trigger is switching from a task that does not use FP (FS=Off) to one that needs FP state restore/initialization, where FP instructions in the switch path can trap before the next task actually runs.

Changes

This PR fixes the issue at the root-cause location in axcpu:

1. src/riscv/context.rs
   • Move sstatus::set_fs(next_fp_state.fs) so it executes before the restore/clear match.
   • This ensures FP instructions are executable even when firmware leaves FS=Off.
   • The conditional table remains the same: FS::Clean → restore(), FS::Initial → clear(), FS::Off → no-op, with FS::Dirty still marked unreachable.

Validation Performed

• Local:
  • cargo fmt --all -- --check passes.
• Behavioral/functional validation is from downstream reproduction:
  • With this fix concept applied, RustSBI boot no longer hits the IllegalInstruction trap in fp-simd switching path.

Impact

• Makes RISC-V fp-simd context switching robust when firmware leaves sstatus.FS=Off.

Files Included In This PR

• src/riscv/context.rs

---

I’m still learning kernel development, so feedback is very welcome.
If any part of this fix should be done differently, I’m happy to revise.