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Subject: Re: [PATCH 17/17] riscv: add nommu support
To:     Christoph Hellwig <hch@lst.de>, Palmer Dabbelt <palmer@sifive.com>
Cc:     Damien Le Moal <damien.lemoal@wdc.com>,
        linux-riscv@lists.infradead.org, uclinux-dev@uclinux.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20190610221621.10938-1-hch@lst.de>
 <20190610221621.10938-18-hch@lst.de>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <f2bb19e7-ce7b-78c0-cafc-15d76003c011@redhat.com>
Date:   Tue, 11 Jun 2019 14:44:12 +0200
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On 11.06.19 00:16, Christoph Hellwig wrote:
> The kernel runs in M-mode without using page tables, and thus doesn't
> can run bare metal without help from additional firmware.

s/doesn't can/can't/

> 
> Most of the patch is just stubbing out code not needed without page
> tables, but there is an interesting detail in the signals implementation:
> 
>  - The normal RISC-V syscall ABI only implements rt_sigreturn as VDSO
>    entry point, but the ELF VDSO is not supported for nommu Linux.
>    We instead copy the code to call the syscall onto the stack.

Interesting read :)

> 
> In addition to enabling the nommu code a new defconfig for a small
> kernel image that can run in nommu mode on qemu is also provided, to run
> a kernel in qemu you can use the following command line:
> 
> qemu-system-riscv64 -smp 2 -m 64 -machine virt -nographic \
> 	-kernel arch/riscv/boot/loader \
> 	-drive file=rootfs.ext2,format=raw,id=hd0 \
> 	-device virtio-blk-device,drive=hd0
> 
> Contains contributions from Damien Le Moal <Damien.LeMoal@wdc.com>.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  arch/riscv/Kconfig                      | 24 +++++---
>  arch/riscv/configs/nommu_virt_defconfig | 78 +++++++++++++++++++++++++
>  arch/riscv/include/asm/elf.h            |  4 +-
>  arch/riscv/include/asm/futex.h          |  6 ++
>  arch/riscv/include/asm/io.h             |  4 ++
>  arch/riscv/include/asm/mmu.h            |  3 +
>  arch/riscv/include/asm/page.h           | 12 +++-
>  arch/riscv/include/asm/pgalloc.h        |  2 +
>  arch/riscv/include/asm/pgtable.h        | 38 ++++++++----
>  arch/riscv/include/asm/tlbflush.h       |  7 ++-
>  arch/riscv/include/asm/uaccess.h        |  4 ++
>  arch/riscv/kernel/Makefile              |  3 +-
>  arch/riscv/kernel/entry.S               | 11 ++++
>  arch/riscv/kernel/head.S                |  6 ++
>  arch/riscv/kernel/signal.c              | 17 +++++-
>  arch/riscv/lib/Makefile                 |  8 +--
>  arch/riscv/mm/Makefile                  |  3 +-
>  arch/riscv/mm/cacheflush.c              |  2 +
>  arch/riscv/mm/context.c                 |  2 +
>  arch/riscv/mm/init.c                    |  2 +
>  20 files changed, 200 insertions(+), 36 deletions(-)
>  create mode 100644 arch/riscv/configs/nommu_virt_defconfig
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 9f2ebd035e80..41edb314ec67 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -27,13 +27,13 @@ config RISCV
>  	select GENERIC_IRQ_SHOW
>  	select GENERIC_PCI_IOMAP
>  	select GENERIC_SCHED_CLOCK
> -	select GENERIC_STRNCPY_FROM_USER
> -	select GENERIC_STRNLEN_USER
> +	select GENERIC_STRNCPY_FROM_USER if MMU
> +	select GENERIC_STRNLEN_USER if MMU
>  	select GENERIC_SMP_IDLE_THREAD
>  	select GENERIC_ATOMIC64 if !64BIT
>  	select HAVE_ARCH_AUDITSYSCALL
>  	select HAVE_MEMBLOCK_NODE_MAP
> -	select HAVE_DMA_CONTIGUOUS
> +	select HAVE_DMA_CONTIGUOUS if MMU
>  	select HAVE_FUTEX_CMPXCHG if FUTEX
>  	select HAVE_PERF_EVENTS
>  	select HAVE_SYSCALL_TRACEPOINTS
> @@ -48,6 +48,7 @@ config RISCV
>  	select PCI_DOMAINS_GENERIC if PCI
>  	select PCI_MSI if PCI
>  	select RISCV_TIMER
> +	select UACCESS_MEMCPY if !MMU
>  	select GENERIC_IRQ_MULTI_HANDLER
>  	select ARCH_HAS_PTE_SPECIAL
>  	select ARCH_HAS_MMIOWB
> @@ -56,9 +57,14 @@ config RISCV
>  # set if we run in machine mode, cleared if we run in supervisor mode
>  config M_MODE
>  	bool
> +	default y if !MMU
>  
>  config MMU
> -	def_bool y
> +	bool "MMU-based Paged Memory Management Support"
> +	default y
> +	help
> +	  Select if you want MMU-based virtualised addressing space
> +	  support by paged memory management. If unsure, say 'Y'.
>  
>  config ZONE_DMA32
>  	bool
> @@ -67,6 +73,7 @@ config ZONE_DMA32
>  config PAGE_OFFSET
>  	hex
>  	default 0xC0000000 if 32BIT && MAXPHYSMEM_2GB
> +	default 0x80000000 if 64BIT && !MMU
>  	default 0xffffffff80000000 if 64BIT && MAXPHYSMEM_2GB
>  	default 0xffffffe000000000 if 64BIT && MAXPHYSMEM_128GB
>  
> @@ -94,7 +101,7 @@ config GENERIC_HWEIGHT
>  	def_bool y
>  
>  config FIX_EARLYCON_MEM
> -	def_bool y
> +	def_bool CONFIG_MMU
>  
>  config PGTABLE_LEVELS
>  	int
> @@ -117,6 +124,7 @@ config ARCH_RV32I
>  	select GENERIC_LIB_ASHRDI3
>  	select GENERIC_LIB_LSHRDI3
>  	select GENERIC_LIB_UCMPDI2
> +	select MMU
>  
>  config ARCH_RV64I
>  	bool "RV64I"
> @@ -125,9 +133,9 @@ config ARCH_RV64I
>  	select HAVE_FUNCTION_TRACER
>  	select HAVE_FUNCTION_GRAPH_TRACER
>  	select HAVE_FTRACE_MCOUNT_RECORD
> -	select HAVE_DYNAMIC_FTRACE
> -	select HAVE_DYNAMIC_FTRACE_WITH_REGS
> -	select SWIOTLB
> +	select HAVE_DYNAMIC_FTRACE if MMU
> +	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
> +	select SWIOTLB if MMU
>  
>  endchoice
>  
> diff --git a/arch/riscv/configs/nommu_virt_defconfig b/arch/riscv/configs/nommu_virt_defconfig
> new file mode 100644
> index 000000000000..cf74e179bf90
> --- /dev/null
> +++ b/arch/riscv/configs/nommu_virt_defconfig
> @@ -0,0 +1,78 @@
> +# CONFIG_CPU_ISOLATION is not set
> +CONFIG_LOG_BUF_SHIFT=16
> +CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT=12
> +CONFIG_BLK_DEV_INITRD=y
> +# CONFIG_RD_BZIP2 is not set
> +# CONFIG_RD_LZMA is not set
> +# CONFIG_RD_XZ is not set
> +# CONFIG_RD_LZO is not set
> +# CONFIG_RD_LZ4 is not set
> +CONFIG_CC_OPTIMIZE_FOR_SIZE=y
> +CONFIG_EXPERT=y
> +# CONFIG_SYSFS_SYSCALL is not set
> +# CONFIG_FHANDLE is not set
> +# CONFIG_BASE_FULL is not set
> +# CONFIG_EPOLL is not set
> +# CONFIG_SIGNALFD is not set
> +# CONFIG_TIMERFD is not set
> +# CONFIG_EVENTFD is not set
> +# CONFIG_AIO is not set
> +# CONFIG_IO_URING is not set
> +# CONFIG_ADVISE_SYSCALLS is not set
> +# CONFIG_MEMBARRIER is not set
> +# CONFIG_KALLSYMS is not set
> +# CONFIG_VM_EVENT_COUNTERS is not set
> +# CONFIG_COMPAT_BRK is not set
> +CONFIG_SLOB=y
> +# CONFIG_SLAB_MERGE_DEFAULT is not set
> +# CONFIG_MMU is not set
> +CONFIG_MAXPHYSMEM_2GB=y
> +CONFIG_SMP=y
> +CONFIG_CMDLINE="root=/dev/vda rw earlycon=uart8250,mmio,0x10000000,115200n8 console=ttyS0"
> +CONFIG_CMDLINE_FORCE=y
> +# CONFIG_BLK_DEV_BSG is not set
> +CONFIG_PARTITION_ADVANCED=y
> +# CONFIG_MSDOS_PARTITION is not set
> +# CONFIG_EFI_PARTITION is not set
> +# CONFIG_MQ_IOSCHED_DEADLINE is not set
> +# CONFIG_MQ_IOSCHED_KYBER is not set
> +CONFIG_BINFMT_FLAT=y
> +# CONFIG_COREDUMP is not set
> +CONFIG_DEVTMPFS=y
> +CONFIG_DEVTMPFS_MOUNT=y
> +# CONFIG_FW_LOADER is not set
> +# CONFIG_ALLOW_DEV_COREDUMP is not set
> +CONFIG_VIRTIO_BLK=y
> +# CONFIG_INPUT_KEYBOARD is not set
> +# CONFIG_INPUT_MOUSE is not set
> +# CONFIG_SERIO is not set
> +# CONFIG_LEGACY_PTYS is not set
> +# CONFIG_LDISC_AUTOLOAD is not set
> +# CONFIG_DEVMEM is not set
> +CONFIG_SERIAL_8250=y
> +# CONFIG_SERIAL_8250_DEPRECATED_OPTIONS is not set
> +CONFIG_SERIAL_8250_CONSOLE=y
> +CONFIG_SERIAL_8250_NR_UARTS=1
> +CONFIG_SERIAL_8250_RUNTIME_UARTS=1
> +CONFIG_SERIAL_OF_PLATFORM=y
> +# CONFIG_HW_RANDOM is not set
> +# CONFIG_HWMON is not set
> +# CONFIG_LCD_CLASS_DEVICE is not set
> +# CONFIG_BACKLIGHT_CLASS_DEVICE is not set
> +# CONFIG_VGA_CONSOLE is not set
> +# CONFIG_HID is not set
> +# CONFIG_USB_SUPPORT is not set
> +CONFIG_VIRTIO_MMIO=y
> +CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES=y
> +CONFIG_SIFIVE_PLIC=y
> +# CONFIG_VALIDATE_FS_PARSER is not set
> +CONFIG_EXT2_FS=y
> +# CONFIG_DNOTIFY is not set
> +# CONFIG_INOTIFY_USER is not set
> +# CONFIG_MISC_FILESYSTEMS is not set
> +CONFIG_LSM="[]"
> +CONFIG_PRINTK_TIME=y
> +# CONFIG_SCHED_DEBUG is not set
> +# CONFIG_RCU_TRACE is not set
> +# CONFIG_FTRACE is not set
> +# CONFIG_RUNTIME_TESTING_MENU is not set
> diff --git a/arch/riscv/include/asm/elf.h b/arch/riscv/include/asm/elf.h
> index ef04084bf0de..d83a4efd052b 100644
> --- a/arch/riscv/include/asm/elf.h
> +++ b/arch/riscv/include/asm/elf.h
> @@ -56,16 +56,16 @@ extern unsigned long elf_hwcap;
>   */
>  #define ELF_PLATFORM	(NULL)
>  
> +#ifdef CONFIG_MMU
>  #define ARCH_DLINFO						\
>  do {								\
>  	NEW_AUX_ENT(AT_SYSINFO_EHDR,				\
>  		(elf_addr_t)current->mm->context.vdso);		\
>  } while (0)
> -
> -
>  #define ARCH_HAS_SETUP_ADDITIONAL_PAGES
>  struct linux_binprm;
>  extern int arch_setup_additional_pages(struct linux_binprm *bprm,
>  	int uses_interp);
> +#endif /* CONFIG_MMU */
>  
>  #endif /* _ASM_RISCV_ELF_H */
> diff --git a/arch/riscv/include/asm/futex.h b/arch/riscv/include/asm/futex.h
> index 4ad6409c4647..418564b96dc4 100644
> --- a/arch/riscv/include/asm/futex.h
> +++ b/arch/riscv/include/asm/futex.h
> @@ -12,6 +12,12 @@
>  #include <linux/errno.h>
>  #include <asm/asm.h>
>  
> +/* We don't even really need the extable code, but for now keep it simple */
> +#ifndef CONFIG_MMU
> +#define __enable_user_access()		do { } while (0)
> +#define __disable_user_access()		do { } while (0)
> +#endif
> +
>  #define __futex_atomic_op(insn, ret, oldval, uaddr, oparg)	\
>  {								\
>  	uintptr_t tmp;						\
> diff --git a/arch/riscv/include/asm/io.h b/arch/riscv/include/asm/io.h
> index fc1189ad3777..d39a8f03e85e 100644
> --- a/arch/riscv/include/asm/io.h
> +++ b/arch/riscv/include/asm/io.h
> @@ -14,6 +14,7 @@
>  #include <linux/types.h>
>  #include <asm/mmiowb.h>
>  
> +#ifdef CONFIG_MMU
>  extern void __iomem *ioremap(phys_addr_t offset, unsigned long size);
>  
>  /*
> @@ -26,6 +27,9 @@ extern void __iomem *ioremap(phys_addr_t offset, unsigned long size);
>  #define ioremap_wt(addr, size) ioremap((addr), (size))
>  
>  extern void iounmap(volatile void __iomem *addr);
> +#else
> +#define pgprot_noncached(x)	(x)
> +#endif /* CONFIG_MMU */
>  
>  /* Generic IO read/write.  These perform native-endian accesses. */
>  #define __raw_writeb __raw_writeb
> diff --git a/arch/riscv/include/asm/mmu.h b/arch/riscv/include/asm/mmu.h
> index 151476fb58cb..967eacb01ab5 100644
> --- a/arch/riscv/include/asm/mmu.h
> +++ b/arch/riscv/include/asm/mmu.h
> @@ -10,6 +10,9 @@
>  #ifndef __ASSEMBLY__
>  
>  typedef struct {
> +#ifndef CONFIG_MMU
> +	unsigned long	end_brk;
> +#endif
>  	void *vdso;
>  #ifdef CONFIG_SMP
>  	/* A local icache flush is needed before user execution can resume. */
> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> index 8ddb6c7fedac..df0256a9a965 100644
> --- a/arch/riscv/include/asm/page.h
> +++ b/arch/riscv/include/asm/page.h
> @@ -78,8 +78,14 @@ typedef struct page *pgtable_t;
>  #define PTE_FMT "%08lx"
>  #endif
>  
> +#ifdef CONFIG_MMU
>  extern unsigned long va_pa_offset;
>  extern unsigned long pfn_base;
> +#define ARCH_PFN_OFFSET		(pfn_base)
> +#else
> +#define va_pa_offset		0
> +#define ARCH_PFN_OFFSET		(PAGE_OFFSET >> PAGE_SHIFT)
> +#endif /* CONFIG_MMU */
>  
>  extern unsigned long max_low_pfn;
>  extern unsigned long min_low_pfn;
> @@ -101,9 +107,7 @@ extern unsigned long min_low_pfn;
>  #define phys_to_page(paddr)	(pfn_to_page(phys_to_pfn(paddr)))
>  
>  #define pfn_valid(pfn) \
> -	(((pfn) >= pfn_base) && (((pfn)-pfn_base) < max_mapnr))
> -
> -#define ARCH_PFN_OFFSET		(pfn_base)
> +	(((pfn) >= ARCH_PFN_OFFSET) && (((pfn) - ARCH_PFN_OFFSET) < max_mapnr))
>  
>  #endif /* __ASSEMBLY__ */
>  
> @@ -115,8 +119,10 @@ extern unsigned long min_low_pfn;
>  #include <asm-generic/memory_model.h>
>  #include <asm-generic/getorder.h>
>  
> +#ifdef CONFIG_MMU
>  /* vDSO support */
>  /* We do define AT_SYSINFO_EHDR but don't use the gate mechanism */
>  #define __HAVE_ARCH_GATE_AREA
> +#endif /* CONFIG_MMU */
>  
>  #endif /* _ASM_RISCV_PAGE_H */
> diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h
> index eb8b0195f27f..8d60c862d2f9 100644
> --- a/arch/riscv/include/asm/pgalloc.h
> +++ b/arch/riscv/include/asm/pgalloc.h
> @@ -10,6 +10,7 @@
>  #include <linux/mm.h>
>  #include <asm/tlb.h>
>  
> +#ifdef CONFIG_MMU
>  static inline void pmd_populate_kernel(struct mm_struct *mm,
>  	pmd_t *pmd, pte_t *pte)
>  {
> @@ -106,6 +107,7 @@ do {                                    \
>  	pgtable_page_dtor(pte);         \
>  	tlb_remove_page((tlb), pte);    \
>  } while (0)
> +#endif /* CONFIG_MMU */
>  
>  static inline void check_pgt_cache(void)
>  {
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index f7c3f7de15f2..dd6a63648e4a 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -24,6 +24,7 @@
>  #include <asm/pgtable-32.h>
>  #endif /* CONFIG_64BIT */
>  
> +#ifdef CONFIG_MMU
>  /* Number of entries in the page global directory */
>  #define PTRS_PER_PGD    (PAGE_SIZE / sizeof(pgd_t))
>  /* Number of entries in the page table */
> @@ -31,7 +32,6 @@
>  
>  /* Number of PGD entries that a user-mode program can use */
>  #define USER_PTRS_PER_PGD   (TASK_SIZE / PGDIR_SIZE)
> -#define FIRST_USER_ADDRESS  0
>  
>  /* Page protection bits */
>  #define _PAGE_BASE	(_PAGE_PRESENT | _PAGE_ACCESSED | _PAGE_USER)
> @@ -81,13 +81,6 @@ extern pgd_t swapper_pg_dir[];
>  #define __S110	PAGE_SHARED_EXEC
>  #define __S111	PAGE_SHARED_EXEC
>  
> -/*
> - * ZERO_PAGE is a global shared page that is always zero,
> - * used for zero-mapped memory areas, etc.
> - */
> -extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
> -#define ZERO_PAGE(vaddr) (virt_to_page(empty_zero_page))
> -
>  static inline int pmd_present(pmd_t pmd)
>  {
>  	return (pmd_val(pmd) & (_PAGE_PRESENT | _PAGE_PROT_NONE));
> @@ -392,6 +385,18 @@ static inline int ptep_clear_flush_young(struct vm_area_struct *vma,
>  #define __pte_to_swp_entry(pte)	((swp_entry_t) { pte_val(pte) })
>  #define __swp_entry_to_pte(x)	((pte_t) { (x).val })
>  
> +#define VMALLOC_SIZE		(KERN_VIRT_SIZE >> 1)
> +#define VMALLOC_END		(PAGE_OFFSET - 1)
> +#define VMALLOC_START		(PAGE_OFFSET - VMALLOC_SIZE)
> +
> +#else /* CONFIG_MMU */
> +
> +#define PAGE_KERNEL		__pgprot(0)
> +#define swapper_pg_dir		NULL
> +#define VMALLOC_START		0
> +
> +#endif /* CONFIG_MMU */
> +
>  #ifdef CONFIG_FLATMEM
>  #define kern_addr_valid(addr)   (1) /* FIXME */
>  #endif
> @@ -404,10 +409,7 @@ static inline void pgtable_cache_init(void)
>  	/* No page table caches to initialize */
>  }
>  
> -#define VMALLOC_SIZE     (KERN_VIRT_SIZE >> 1)
> -#define VMALLOC_END      (PAGE_OFFSET - 1)
> -#define VMALLOC_START    (PAGE_OFFSET - VMALLOC_SIZE)
> -
> +#ifdef CONFIG_MMU
>  /*
>   * Task size is 0x40000000000 for RV64 or 0xb800000 for RV32.
>   * Note that PGDIR_SIZE must evenly divide TASK_SIZE.
> @@ -417,6 +419,18 @@ static inline void pgtable_cache_init(void)
>  #else
>  #define TASK_SIZE VMALLOC_START
>  #endif
> +#else /* CONFIG_MMU */
> +#define TASK_SIZE 0xffffffffUL
> +#endif /* !CONFIG_MMU */
> +
> +#define FIRST_USER_ADDRESS  0
> +
> +/*
> + * ZERO_PAGE is a global shared page that is always zero,
> + * used for zero-mapped memory areas, etc.
> + */
> +extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
> +#define ZERO_PAGE(vaddr) (virt_to_page(empty_zero_page))
>  
>  #include <asm-generic/pgtable.h>
>  
> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> index 687dd19735a7..f127b3d17ec2 100644
> --- a/arch/riscv/include/asm/tlbflush.h
> +++ b/arch/riscv/include/asm/tlbflush.h
> @@ -10,6 +10,7 @@
>  #include <linux/mm_types.h>
>  #include <asm/smp.h>
>  
> +#ifdef CONFIG_MMU
>  /*
>   * Flush entire local TLB.  'sfence.vma' implicitly fences with the instruction
>   * cache as well, so a 'fence.i' is not necessary.
> @@ -24,8 +25,12 @@ static inline void local_flush_tlb_page(unsigned long addr)
>  {
>  	__asm__ __volatile__ ("sfence.vma %0" : : "r" (addr) : "memory");
>  }
> +#else /* CONFIG_MMU */
> +#define local_flush_tlb_all()			do { } while (0)
> +#define local_flush_tlb_page(addr)		do { } while (0)
> +#endif /* CONFIG_MMU */
>  
> -#ifndef CONFIG_SMP
> +#if !defined(CONFIG_SMP) || !defined(CONFIG_MMU)
>  
>  #define flush_tlb_all() local_flush_tlb_all()
>  #define flush_tlb_page(vma, addr) local_flush_tlb_page(addr)
> diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
> index e076437cfafe..f462a183a9c2 100644
> --- a/arch/riscv/include/asm/uaccess.h
> +++ b/arch/riscv/include/asm/uaccess.h
> @@ -11,6 +11,7 @@
>  /*
>   * User space memory access functions
>   */
> +#ifdef CONFIG_MMU
>  #include <linux/errno.h>
>  #include <linux/compiler.h>
>  #include <linux/thread_info.h>
> @@ -475,4 +476,7 @@ unsigned long __must_check clear_user(void __user *to, unsigned long n)
>  	__ret;							\
>  })
>  
> +#else /* CONFIG_MMU */
> +#include <asm-generic/uaccess.h>
> +#endif /* CONFIG_MMU */
>  #endif /* _ASM_RISCV_UACCESS_H */
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index f933c04f89db..d720666283d6 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -25,9 +25,8 @@ obj-y	+= time.o
>  obj-y	+= traps.o
>  obj-y	+= riscv_ksyms.o
>  obj-y	+= stacktrace.o
> -obj-y	+= vdso.o
>  obj-y	+= cacheinfo.o
> -obj-y	+= vdso/
> +obj-$(CONFIG_MMU) += vdso.o vdso/
>  
>  obj-$(CONFIG_M_MODE)		+= clint.o
>  obj-$(CONFIG_FPU)		+= fpu.o
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 18eeb1eec9d5..953c64b0b2b9 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -370,6 +370,10 @@ ENTRY(__switch_to)
>  	ret
>  ENDPROC(__switch_to)
>  
> +#ifndef CONFIG_MMU
> +#define do_page_fault do_trap_unknown
> +#endif
> +
>  	.section ".rodata"
>  	/* Exception vector table */
>  ENTRY(excp_vect_table)
> @@ -391,3 +395,10 @@ ENTRY(excp_vect_table)
>  	RISCV_PTR do_page_fault   /* store page fault */
>  excp_vect_table_end:
>  END(excp_vect_table)
> +
> +#ifndef CONFIG_MMU
> +ENTRY(__user_rt_sigreturn)
> +	li a7, __NR_rt_sigreturn
> +	scall
> +END(__user_rt_sigreturn)
> +#endif
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index e05379fd8b64..d869cc7c6a12 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -68,10 +68,12 @@ clear_bss_done:
>  	la a2, boot_cpu_hartid
>  	REG_S a0, (a2)
>  
> +#ifdef CONFIG_MMU
>  	/* Initialize page tables and relocate to virtual addresses */
>  	la sp, init_thread_union + THREAD_SIZE
>  	call setup_vm
>  	call relocate
> +#endif /* CONFIG_MMU */
>  
>  	/* Restore C environment */
>  	la tp, init_task
> @@ -83,6 +85,7 @@ clear_bss_done:
>  	call parse_dtb
>  	tail start_kernel
>  
> +#ifdef CONFIG_MMU
>  relocate:
>  	/* Relocate return address */
>  	li a1, PAGE_OFFSET
> @@ -134,6 +137,7 @@ relocate:
>  	sfence.vma
>  
>  	ret
> +#endif /* CONFIG_MMU */
>  
>  .Lsecondary_start:
>  #ifdef CONFIG_SMP
> @@ -162,8 +166,10 @@ relocate:
>  	beqz tp, .Lwait_for_cpu_up
>  	fence
>  
> +#ifdef CONFIG_MMU
>  	/* Enable virtual memory and relocate to virtual address */
>  	call relocate
> +#endif
>  
>  	tail smp_callin
>  #endif
> diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
> index dfb26b64e17a..7de7e539b25b 100644
> --- a/arch/riscv/kernel/signal.c
> +++ b/arch/riscv/kernel/signal.c
> @@ -17,11 +17,16 @@
>  #include <asm/switch_to.h>
>  #include <asm/csr.h>
>  
> +extern u32 __user_rt_sigreturn[2];
> +
>  #define DEBUG_SIG 0
>  
>  struct rt_sigframe {
>  	struct siginfo info;
>  	struct ucontext uc;
> +#ifndef CONFIG_MMU
> +	u32 sigreturn_code[2];
> +#endif
>  };
>  
>  #ifdef CONFIG_FPU
> @@ -166,7 +171,6 @@ static inline void __user *get_sigframe(struct ksignal *ksig,
>  	return (void __user *)sp;
>  }
>  
> -
>  static int setup_rt_frame(struct ksignal *ksig, sigset_t *set,
>  	struct pt_regs *regs)
>  {
> @@ -189,8 +193,19 @@ static int setup_rt_frame(struct ksignal *ksig, sigset_t *set,
>  		return -EFAULT;
>  
>  	/* Set up to return from userspace. */
> +#ifdef CONFIG_MMU
>  	regs->ra = (unsigned long)VDSO_SYMBOL(
>  		current->mm->context.vdso, rt_sigreturn);
> +#else
> +	/*
> +	 * For the nommu case we don't have a VDSO.  Instead we push two
> +	 * instructions to call the rt_sigreturn syscall onto the user stack.
> +	 */
> +	if (copy_to_user(&frame->sigreturn_code, __user_rt_sigreturn,
> +			sizeof(frame->sigreturn_code)))
> +		return -EFAULT;
> +	regs->ra = (unsigned long)&frame->sigreturn_code;;
> +#endif /* CONFIG_MMU */
>  
>  	/*
>  	 * Set up registers for signal handler.
> diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
> index 8e364ebf37de..5382e42d0cba 100644
> --- a/arch/riscv/lib/Makefile
> +++ b/arch/riscv/lib/Makefile
> @@ -2,8 +2,6 @@
>  lib-y	+= delay.o
>  lib-y	+= memcpy.o
>  lib-y	+= memset.o
> -lib-y	+= uaccess.o
> -
> -lib-$(CONFIG_64BIT) += tishift.o
> -
> -lib-$(CONFIG_32BIT) += udivdi3.o
> +lib-$(CONFIG_MMU)	+= uaccess.o
> +lib-$(CONFIG_64BIT)	+= tishift.o
> +lib-$(CONFIG_32BIT)	+= udivdi3.o
> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
> index fc51d3b7876e..8fc2fd148b06 100644
> --- a/arch/riscv/mm/Makefile
> +++ b/arch/riscv/mm/Makefile
> @@ -6,9 +6,8 @@ CFLAGS_REMOVE_init.o = -pg
>  endif
>  
>  obj-y += init.o
> -obj-y += fault.o
>  obj-y += extable.o
> -obj-y += ioremap.o
> +obj-$(CONFIG_MMU) += fault.o ioremap.o
>  obj-y += cacheflush.o
>  obj-y += context.o
>  obj-y += sifive_l2_cache.o
> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> index 10875ea1065e..0fc69eedc921 100644
> --- a/arch/riscv/mm/cacheflush.c
> +++ b/arch/riscv/mm/cacheflush.c
> @@ -90,6 +90,7 @@ void flush_icache_mm(struct mm_struct *mm, bool local)
>  
>  #endif /* CONFIG_SMP */
>  
> +#ifdef CONFIG_MMU
>  void flush_icache_pte(pte_t pte)
>  {
>  	struct page *page = pte_page(pte);
> @@ -97,3 +98,4 @@ void flush_icache_pte(pte_t pte)
>  	if (!test_and_set_bit(PG_dcache_clean, &page->flags))
>  		flush_icache_all();
>  }
> +#endif /* CONFIG_MMU */
> diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
> index beeb5d7f92ea..073ff12a838a 100644
> --- a/arch/riscv/mm/context.c
> +++ b/arch/riscv/mm/context.c
> @@ -57,8 +57,10 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>  	cpumask_clear_cpu(cpu, mm_cpumask(prev));
>  	cpumask_set_cpu(cpu, mm_cpumask(next));
>  
> +#ifdef CONFIG_MMU
>  	csr_write(CSR_SATP, virt_to_pfn(next->pgd) | SATP_MODE);
>  	local_flush_tlb_all();
> +#endif
>  
>  	flush_icache_deferred(next);
>  }
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 84747d7a1e85..782be273e714 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -142,6 +142,7 @@ void __init setup_bootmem(void)
>  	}
>  }
>  
> +#ifdef CONFIG_MMU
>  unsigned long va_pa_offset;
>  EXPORT_SYMBOL(va_pa_offset);
>  unsigned long pfn_base;
> @@ -247,3 +248,4 @@ asmlinkage void __init setup_vm(void)
>  				__pgprot(_PAGE_TABLE));
>  #endif
>  }
> +#endif /* CONFIG_MMU */
> 


-- 

Thanks,

David / dhildenb