[PATCH 1/1] riscv: fix NR_CPUS range leaking 64-bit default onto 32-bit builds

[PATCH 1/1] riscv: fix NR_CPUS range leaking 64-bit default onto 32-bit builds

[Leo Yu-Chi Liang]

rv32_defconfig is built in two stages:

- the generic (64-bit-oriented) "defconfig" target runs first
  and resolves a concrete NR_CPUS value using the 64BIT default.
- then 32-bit.config is merged on top via merge_config.sh,
  which does not touch NR_CPUS.

The final "make olddefconfig" pass sees NR_CPUS=256 is still within
the "2 512" range therefore never recalculates it against the 32BIT
default of 32, silently keeping the leaked 64-bit-stage value.

The NR_CPUS-scaled regions (percpu areas, ...etc) of the resulting
CONFIG_NR_CPUS=256 on a 32-bit build bloats and does not fit in the
percpu/stack layout for rv32 and hangs early boot on QEMU's virt machine.

Fix this by explicitly specify the range for 32/64 and SBI_V01/!SBI_V01.

Fixes: 0fa749771993 ("riscv: Raise default NR_CPUS for 64BIT to 256")
Signed-off-by: Leo Yu-Chi Liang <leo.liang@sifive.com>
---
 arch/riscv/Kconfig | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d6c2dbf8455..5874ca30f4f 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -453,9 +453,10 @@ config SMP
 config NR_CPUS
 	int "Maximum number of CPUs (2-512)"
 	depends on SMP
-	range 2 512 if !RISCV_SBI_V01
-	range 2 32 if RISCV_SBI_V01 && 32BIT
+	range 2 512 if !RISCV_SBI_V01 && 64BIT
+	range 2 128 if !RISCV_SBI_V01 && 32BIT
 	range 2 64 if RISCV_SBI_V01 && 64BIT
+	range 2 32 if RISCV_SBI_V01 && 32BIT
 	default "32" if 32BIT
 	default "64" if RISCV_SBI_V01 && 64BIT
 	default "256" if !RISCV_SBI_V01 && 64BIT
-- 
2.43.7

Re: [PATCH 1/1] riscv: fix NR_CPUS range leaking 64-bit default onto 32-bit builds

[Paul Walmsley]

On Tue, 6 Oct 2026, Leo Yu-Chi Liang wrote:

> rv32_defconfig is built in two stages:
> 
> - the generic (64-bit-oriented) "defconfig" target runs first
>   and resolves a concrete NR_CPUS value using the 64BIT default.
> - then 32-bit.config is merged on top via merge_config.sh,
>   which does not touch NR_CPUS.
> 
> The final "make olddefconfig" pass sees NR_CPUS=256 is still within
> the "2 512" range therefore never recalculates it against the 32BIT
> default of 32, silently keeping the leaked 64-bit-stage value.
> 
> The NR_CPUS-scaled regions (percpu areas, ...etc) of the resulting
> CONFIG_NR_CPUS=256 on a 32-bit build bloats and does not fit in the
> percpu/stack layout for rv32 and hangs early boot on QEMU's virt machine.
> 
> Fix this by explicitly specify the range for 32/64 and SBI_V01/!SBI_V01.
> 
> Fixes: 0fa749771993 ("riscv: Raise default NR_CPUS for 64BIT to 256")
> Signed-off-by: Leo Yu-Chi Liang <leo.liang@sifive.com>

Thanks, queued for v7.3-rc.


- Paul