[RFC] OpenSBI: Lazy floating-point and vector context switching

[Vivian Wang]

On 3/12/26 01:11, dave.patel at riscstar.com wrote:
> Hi OpenSBI maintainers and community,
>
> I am working on an implementation to support floating-point (FP) and
> vector register context management during domain and application
> context switches in OpenSBI.
>
> Currently, OpenSBI saves and restores only the integer register
> context when handling traps or domain transitions. The FP registers
> (f0–f31) and vector registers (v0–v31) are not currently managed by
> OpenSBI.
>
> With increasing adoption of the RISC-V vector extension and
> floating-point workloads, it becomes important to ensure correct
> behavior when S-mode software or trusted applications use these
> execution units.
>
> This email proposes an approach to support lazy FP and vector
> context switching, aligned with the mechanism described in the
> RISC-V privileged specification.
>
> ---
>
> ## Background
>
> In RISC-V, the `FS` (Floating-point State) and `VS` (Vector State)
> fields in `mstatus` / `sstatus` control access to the floating-point
> and vector units.
>
> When these fields are set to `Off`, any attempt to execute FP or
> vector instructions results in an illegal instruction trap.
>
> This mechanism allows operating systems and firmware to implement
> lazy context switching where FP/vector registers are saved or
> restored only when they are actually used.

This does not work in Machine mode, where OpenSBI resides.

See privileged spec, section "Extension Context Status in mstatus
Register": "Machine and Supervisor modes share a single copy of the FS,
VS, and XS bits."

This means that a Supervisor mode program can set sstatus.FS to Dirty,
access floating point state, and set sstatus.FS back to Off without
Machine mode software noticing ever.

See also https://github.com/riscv/riscv-isa-manual/issues/832
"Impossible to implement HS-level sstatus.{FS,VS} = Off with
software-emulated hypervisor extension?" where I ran into the same
problem in a different context.

So basically, lazy saving/restoring F and V in Machine mode with
multiple Supervisor mode programs is not a designed feature, and is not
possible as the privileged architecture is currently designed. For these
use cases non-lazy switching is required.

Vivian "dramforever" Wang