Re: [PATCH v3 3/3] lib: sbi: ISA extension emulation (Zvbb).

Hi Benedikt,

We discovered another subtle problem while testing Zvbb emulation:

On 12/27/25 20:18, Benedikt Freisen wrote:
> Add trap-based emulation code for the ISA extension Zvbb based on RVV 1.0 and Zbb.
>
> Signed-off-by: Benedikt Freisen <[email protected]>
> ---
>
> [...]
>
> diff --git a/lib/sbi/sbi_insn_emu_v.c b/lib/sbi/sbi_insn_emu_v.c
> new file mode 100644
> index 00000000..483b0ee6
> --- /dev/null
> +++ b/lib/sbi/sbi_insn_emu_v.c
>
> [...]
>
> +
> +int sbi_insn_emu_op_v(ulong insn, struct sbi_trap_regs *regs)
> +{
>
>
> [...]
>
> +
> +	regs->mepc += 4;
> +
> +	return 0;
> +}
> +

sbi_insn_emu_op_v does not set xstatus.VS to dirty, which has a chance
to break context switching and cause vector register state to be broken.
Something like SET_VS_DIRTY from [1] is needed before returning:

#define SET_VS_DIRTY(regs) do {				\
	if (sbi_regs_from_virt(regs))			\
		csr_set(CSR_VSSTATUS, MSTATUS_VS);	\
	regs->mstatus |= MSTATUS_VS;			\
} while(0)

It seems unlikely that upstream OpenSBI would accept these emulation
patches, but I'm reporting this for the sake of anyone potentially
running into this.

Thanks,
Vivian "dramforever" Wang

[1]: https://lore.kernel.org/opensbi/[email protected]/


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