poemonsense · Sep 21, 2026 · Sep 21, 2026
diff --git a/Makefile b/Makefile
@@ -47,6 +47,9 @@ endif
 # from it through the print-buildroot-version target, so a bump here is enough.
 BUILDROOT_VERSION := 2026.08
 BUILDROOT_DIR := build/buildroot
+# DEFAULT_DTB=nutshell or nutshell-* -> nutshell_defconfig; otherwise xiangshan_defconfig.
+BUILDROOT_DEFCONFIG := $(if $(filter nutshell nutshell-%,$(DEFAULT_DTB)),nutshell_defconfig,xiangshan_defconfig)
+BUILDROOT_CONFIG_STAMP := build/buildroot-config/$(BUILDROOT_DEFCONFIG)
 $(BUILDROOT_DIR)/Makefile:
 	mkdir -p build
 	wget https://buildroot.org/downloads/buildroot-$(BUILDROOT_VERSION).tar.gz -O build/buildroot.tar.gz
@@ -56,17 +59,22 @@ $(BUILDROOT_DIR)/Makefile:
 print-buildroot-version:
 	@echo $(BUILDROOT_VERSION)
 
+$(BUILDROOT_CONFIG_STAMP): br2-external/configs/$(BUILDROOT_DEFCONFIG) $(BUILDROOT_DIR)/Makefile
+	mkdir -p "$(@D)"
+	rm -f build/buildroot-config/*
+	touch $@
+
 # Prepare buildroot SDK
 TOOLCHAIN_WRAPPER := $(BUILDROOT_DIR)/output/host/bin/toolchain-wrapper
-$(TOOLCHAIN_WRAPPER): br2-external/configs/nemu_defconfig $(BUILDROOT_DIR)/Makefile
-	$(MAKE) -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external) nemu_defconfig
+$(TOOLCHAIN_WRAPPER): $(BUILDROOT_CONFIG_STAMP)
+	$(MAKE) -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external) $(BUILDROOT_DEFCONFIG)
 	$(MAKE) -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external) prepare-sdk
 	touch $(TOOLCHAIN_WRAPPER)
 
 # Build Linux kernel
 LINUX_IMAGE := $(BUILDROOT_DIR)/output/images/Image
-$(LINUX_IMAGE): $(TOOLCHAIN_WRAPPER) br2-external/configs/nemu_defconfig br2-external/board/openxiangshan/nemu/linux.config
-	$(MAKE) -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external) nemu_defconfig
+$(LINUX_IMAGE): $(TOOLCHAIN_WRAPPER) $(BUILDROOT_CONFIG_STAMP) br2-external/board/openxiangshan/nemu/linux.config $(if $(filter nutshell_defconfig,$(BUILDROOT_DEFCONFIG)),br2-external/board/openxiangshan/nemu/linux-nutshell.fragment)
+	$(MAKE) -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external) $(BUILDROOT_DEFCONFIG)
 	$(MAKE) -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external)
 
 # Build GCPT. Single-core firmware keeps LibCheckpointAlpha; LibCheckpoint is
@@ -217,7 +225,7 @@ prepare-sdk: $(TOOLCHAIN_WRAPPER)
 
 # Download all source files needed by buildroot
 source: $(BUILDROOT_DIR)/Makefile
-	make -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external) nemu_defconfig
+	make -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external) $(BUILDROOT_DEFCONFIG)
 	make -C $(BUILDROOT_DIR) BR2_EXTERNAL=$(abspath br2-external) source
 
 # Build all all-in-one firmware images

diff --git a/README.md b/README.md
@@ -114,13 +114,20 @@ builds default `GCPT_SERIAL_PORT` to the QEMU 16550A TX register at
 ## Workload Compatibility
 
 Not all workloads can run on all NEMU or QEMU configurations. QEMU support is
-limited to `linux/coremark`, `linux/spec2006`, and `linux/spec2017`; the only workload supported by
-`riscv64-nutshell_defconfig` is `linux/hello`, since all other workloads require
-hardware floating point, which is not supported by nutshell. RVV related workloads
-require the vector ISA extension, and hypervisor related workloads require the
-hypervisor ISA extension.
+limited to `linux/coremark`, `linux/spec2006`, and `linux/spec2017`. RVV related
+workloads require the vector ISA extension, and hypervisor related workloads
+require the hypervisor ISA extension.
 
-NutShell uses the RV64IMAC/LP64 software stack. See the
+`xiangshan_defconfig` is the XiangShan/NEMU Buildroot tree. NutShell is RV64IMAC;
+Linux images use `nutshell_defconfig` (lp64) when `DEFAULT_DTB` is `nutshell`,
+`nutshell-mem2g`, or `nutshell-fpga`:
+
+```sh
+make linux/hello DEFAULT_DTB=nutshell
+make linux/spec2006 BENCH=mcf DEFAULT_DTB=nutshell-mem2g SPEC2006_ISO=/path/to/cpu2006.iso
+```
+
+See the
 [SPEC2006 configuration](workloads/linux/spec2006/README.md#configuration)
 section for `riscv_gcc16_rv64imac_lp64.cfg`.
 

diff --git a/br2-external/board/openxiangshan/nemu/linux-nutshell.fragment b/br2-external/board/openxiangshan/nemu/linux-nutshell.fragment
@@ -0,0 +1,6 @@
+# NutShell is RV64IMAC. Share linux.config with XiangShan; turn off ISA
+# the core does not implement. linux.config enables KVM.
+# CONFIG_FPU is not set
+# CONFIG_RISCV_ISA_V is not set
+# CONFIG_VIRTUALIZATION is not set
+# CONFIG_KVM is not set
diff --git a/br2-external/configs/nutshell_defconfig b/br2-external/configs/nutshell_defconfig
@@ -0,0 +1,22 @@
+# NutShell RV64IMAC/LP64. Bootlin only ships lp64d, so Buildroot builds gcc.
+# Keep xiangshan_defconfig for XiangShan/NEMU.
+BR2_riscv=y
+BR2_riscv_custom=y
+BR2_RISCV_ISA_RVM=y
+BR2_RISCV_ISA_RVA=y
+BR2_RISCV_ISA_RVC=y
+BR2_RISCV_64=y
+BR2_RISCV_ABI_LP64=y
+BR2_TOOLCHAIN_BUILDROOT=y
+BR2_TOOLCHAIN_BUILDROOT_GLIBC=y
+BR2_TARGET_GENERIC_HOSTNAME="nutshell"
+BR2_TARGET_GENERIC_ISSUE="NutShell rv64imac/lp64"
+BR2_TARGET_GENERIC_ROOT_PASSWD="xiangshan"
+BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
+BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="${BR2_EXTERNAL}/board/openxiangshan/nemu/linux.config"
+BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="${BR2_EXTERNAL}/board/openxiangshan/nemu/linux-nutshell.fragment"
+BR2_TARGET_ROOTFS_INITRAMFS=y
+BR2_PACKAGE_BUSYBOX_CONFIG_FRAGMENT_FILES="${BR2_EXTERNAL}/board/openxiangshan/nemu/busybox-taskset.fragment"
+BR2_PACKAGE_NEMU_TRAP=y
+BR2_PACKAGE_NEMU_EXEC=y
diff --git a/br2-external/configs/nemu_defconfig → br2-external/configs/xiangshan_defconfig b/br2-external/configs/nemu_defconfig → br2-external/configs/xiangshan_defconfig
@@ -1,3 +1,4 @@
+# XiangShan/NEMU (Bootlin rv64gc). NutShell uses nutshell_defconfig.
 BR2_riscv=y
 BR2_RISCV_ISA_RVC=y
 BR2_TOOLCHAIN_EXTERNAL=y

diff --git a/dts/README.md b/dts/README.md
@@ -41,6 +41,9 @@ profiles, FPGA noAIA names, and QEMU `nemu` names of the form
 custom template passed through `--custom-template-dir`; otherwise generation
 fails with an `unsupported DTS basename` error.
 
+Linux images with `DEFAULT_DTB=nutshell` or `nutshell-*` use
+`nutshell_defconfig` (RV64IMAC/LP64) instead of `xiangshan_defconfig`.
+
 `nutshell` is the 128 MiB machine description used by NEMU's
 `riscv64-nutshell_defconfig`: uartlite at `0x40600000`, no PLIC.
 NutShell simulation uses the same map. `nutshell-mem2g` keeps that

diff --git a/workloads/linux/spec2006/README.md b/workloads/linux/spec2006/README.md
@@ -220,12 +220,16 @@ For GCC 15, select:
 workloads/linux/spec2006/configs/riscv_gcc15_base.cfg
 ```
 
-For NutShell, select the RV64IMAC/lp64 configuration:
+For NutShell (`DEFAULT_DTB=nutshell`, `nutshell-mem2g`, or `nutshell-fpga`),
+the default is the RV64IMAC/lp64 configuration:
 
 ```text
 workloads/linux/spec2006/configs/riscv_gcc16_rv64imac_lp64.cfg
 ```
 
+That compiler must be an rv64imac/lp64 `riscv64-unknown-linux-gnu-gcc`, not
+the XiangShan RVA23 toolchain that uses the same triple.
+
 Override it with:
 
 ```sh

diff --git a/workloads/linux/spec2006/rules.mk b/workloads/linux/spec2006/rules.mk
@@ -7,7 +7,7 @@ SPEC2006_SCRIPTS_DIR := $(SPEC2006_REPO_ROOT)/scripts
 SPEC2006_DTS_DIR := $(SPEC2006_REPO_ROOT)/build/generated-dts
 SPEC2006_BUILD_DIR ?= $(SPEC2006_REPO_ROOT)/build/linux-workloads/spec2006
 SPEC2006_CASE_CONFIG := $(SPEC2006_WORKLOAD_DIR)/spec06.json
-SPEC2006_CFG ?= $(SPEC2006_WORKLOAD_DIR)/configs/gcc16-linux-riscv64-rva23u64_novec.cfg
+SPEC2006_CFG ?= $(SPEC2006_WORKLOAD_DIR)/configs/$(if $(filter nutshell nutshell-%,$(DEFAULT_DTB)),riscv_gcc16_rv64imac_lp64.cfg,gcc16-linux-riscv64-rva23u64_novec.cfg)
 SPEC2006_HELPER := $(SPEC2006_WORKLOAD_DIR)/spec2006-package.py
 QEMU_DEFAULT_DTB ?= xiangshan-qemu-nemu-mem2g
 SPEC2006_SOURCE_SPEC_ISO := $(SPEC2006_ISO)