[PATCH v23 24/28] arch/riscv: dual vdso creation logic and select vdso based on hw

[Thomas Weißschuh]

On Wed, Nov 12, 2025 at 04:43:22PM -0800, Deepak Gupta via B4 Relay wrote:
> From: Deepak Gupta <debug at rivosinc.com>
> 
> Shadow stack instructions are taken from zimop (mandated on RVA23).
> Any hardware prior to RVA23 profile will fault on shadow stack instruction.
> Any userspace with shadow stack instruction in it will fault on such
> hardware. Thus such userspace can't be brought onto such a hardware.
> 
> It's not known how userspace will respond to such binary fragmentation.
> However in order to keep kernel portable across such different hardware,
> `arch/riscv/kernel/vdso_cfi` is created which has logic (Makefile) to
> compile `arch/riscv/kernel/vdso` sources with cfi flags and then changes
> in `arch/riscv/kernel/vdso.c` for selecting appropriate vdso depending
> on whether underlying hardware(cpu) implements zimop extension. Offset
> of vdso symbols will change due to having two different vdso binaries,
> there is added logic to include new generated vdso offset header and
> dynamically select offset (like for rt_sigreturn).

If the used vDSO variant only depends on the hardware and nothing else,
why not use alternative patching and avoid the complexity?
I see that RISCV_ALTERNATIVE depends on !XIP_KERNEL but the vDSO code is
moved to dynamically allocated memory in any case, so it is patchable.

> Signed-off-by: Deepak Gupta <debug at rivosinc.com>
> Acked-by: Charles Mirabile <cmirabil at redhat.com>

(...)