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In-Reply-To: <20191017173743.5430-14-hch@lst.de>
From:   Anup Patel <anup@brainfault.org>
Date:   Fri, 18 Oct 2019 08:34:36 +0530
Message-ID: <CAAhSdy0CgYkbov+ZACOxE1ReOBEthT96mwq=WzY0feK_NaA1-A@mail.gmail.com>
Subject: Re: [PATCH 13/15] riscv: add nommu support
To:     Christoph Hellwig <hch@lst.de>
Cc:     Palmer Dabbelt <palmer@sifive.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Damien Le Moal <damien.lemoal@wdc.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Oct 17, 2019 at 11:08 PM Christoph Hellwig <hch@lst.de> wrote:
>
> The kernel runs in M-mode without using page tables, and thus can't run
> bare metal without help from additional firmware.
>
> 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.
>
> 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                      | 26 ++++++---
>  arch/riscv/configs/nommu_virt_defconfig | 78 +++++++++++++++++++++++++
>  arch/riscv/include/asm/cache.h          |  8 +++
>  arch/riscv/include/asm/elf.h            |  4 +-
>  arch/riscv/include/asm/fixmap.h         |  2 +
>  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           | 10 +++-
>  arch/riscv/include/asm/pgalloc.h        |  2 +
>  arch/riscv/include/asm/pgtable.h        | 64 +++++++++++---------
>  arch/riscv/include/asm/tlbflush.h       | 12 +++-
>  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                 | 11 ++--
>  arch/riscv/mm/Makefile                  |  3 +-
>  arch/riscv/mm/cacheflush.c              |  2 +
>  arch/riscv/mm/context.c                 |  2 +
>  arch/riscv/mm/init.c                    | 13 ++++-
>  22 files changed, 236 insertions(+), 55 deletions(-)
>  create mode 100644 arch/riscv/configs/nommu_virt_defconfig
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index b85492c42ccb..babc8a0d3d2e 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -26,14 +26,14 @@ 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_ASM_MODVERSIONS
>         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_PERF_REGS
> @@ -50,6 +50,7 @@ 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 GENERIC_ARCH_TOPOLOGY if SMP
>         select ARCH_HAS_PTE_SPECIAL
> @@ -60,7 +61,7 @@ config RISCV
>         select ARCH_WANT_HUGE_PMD_SHARE if 64BIT
>         select SPARSEMEM_STATIC if 32BIT
>         select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
> -       select HAVE_ARCH_MMAP_RND_BITS
> +       select HAVE_ARCH_MMAP_RND_BITS if MMU
>
>  config ARCH_MMAP_RND_BITS_MIN
>         default 18 if 64BIT
> @@ -75,6 +76,7 @@ config ARCH_MMAP_RND_BITS_MAX
>  # set if we run in machine mode, cleared if we run in supervisor mode
>  config RISCV_M_MODE
>         bool
> +       default !MMU
>
>  # set if we are running in S-mode and can use SBI calls
>  config RISCV_SBI
> @@ -83,7 +85,11 @@ config RISCV_SBI
>         default y
>
>  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
> @@ -102,6 +108,7 @@ config PA_BITS
>  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
>
> @@ -145,7 +152,7 @@ config GENERIC_HWEIGHT
>         def_bool y
>
>  config FIX_EARLYCON_MEM
> -       def_bool y
> +       def_bool CONFIG_MMU
>
>  config PGTABLE_LEVELS
>         int
> @@ -170,6 +177,7 @@ config ARCH_RV32I
>         select GENERIC_LIB_ASHRDI3
>         select GENERIC_LIB_LSHRDI3
>         select GENERIC_LIB_UCMPDI2
> +       select MMU
>
>  config ARCH_RV64I
>         bool "RV64I"
> @@ -178,9 +186,9 @@ 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/cache.h b/arch/riscv/include/asm/cache.h
> index bfd523e8f0b2..9b58b104559e 100644
> --- a/arch/riscv/include/asm/cache.h
> +++ b/arch/riscv/include/asm/cache.h
> @@ -11,4 +11,12 @@
>
>  #define L1_CACHE_BYTES         (1 << L1_CACHE_SHIFT)
>
> +/*
> + * RISC-V requires the stack pointer to be 16-byte aligned, so ensure that
> + * the flat loader aligns it accordingly.
> + */
> +#ifndef CONFIG_MMU
> +#define ARCH_SLAB_MINALIGN     16
> +#endif
> +
>  #endif /* _ASM_RISCV_CACHE_H */
> 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/fixmap.h b/arch/riscv/include/asm/fixmap.h
> index 161f28d04a07..42d2c42f3cc9 100644
> --- a/arch/riscv/include/asm/fixmap.h
> +++ b/arch/riscv/include/asm/fixmap.h
> @@ -11,6 +11,7 @@
>  #include <asm/page.h>
>  #include <asm/pgtable.h>
>
> +#ifdef CONFIG_MMU
>  /*
>   * Here we define all the compile-time 'special' virtual addresses.
>   * The point is to have a constant address at compile time, but to
> @@ -42,4 +43,5 @@ extern void __set_fixmap(enum fixed_addresses idx,
>
>  #include <asm-generic/fixmap.h>
>
> +#endif /* CONFIG_MMU */
>  #endif /* _ASM_RISCV_FIXMAP_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 3db261c4810f..ac699246ae7e 100644
> --- a/arch/riscv/include/asm/page.h
> +++ b/arch/riscv/include/asm/page.h
> @@ -88,8 +88,14 @@ 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;
> @@ -112,11 +118,9 @@ extern unsigned long min_low_pfn;
>
>  #ifdef CONFIG_FLATMEM
>  #define pfn_valid(pfn) \
> -       (((pfn) >= pfn_base) && (((pfn)-pfn_base) < max_mapnr))
> +       (((pfn) >= ARCH_PFN_OFFSET) && (((pfn) - ARCH_PFN_OFFSET) < max_mapnr))
>  #endif
>
> -#define ARCH_PFN_OFFSET                (pfn_base)
> -
>  #endif /* __ASSEMBLY__ */
>
>  #define virt_addr_valid(vaddr) (pfn_valid(virt_to_pfn(vaddr)))
> diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h
> index d59ea92285ec..3f601ee8233f 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
>  #include <asm-generic/pgalloc.h>       /* for pte_{alloc,free}_one */
>
>  static inline void pmd_populate_kernel(struct mm_struct *mm,
> @@ -81,5 +82,6 @@ do {                                    \
>         pgtable_pte_page_dtor(pte);     \
>         tlb_remove_page((tlb), pte);    \
>  } while (0)
> +#endif /* CONFIG_MMU */
>
>  #endif /* _ASM_RISCV_PGALLOC_H */
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 42292d99cc74..f3636fc22a11 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)
> @@ -83,10 +83,6 @@ extern pgd_t swapper_pg_dir[];
>  #define __S110 PAGE_SHARED_EXEC
>  #define __S111 PAGE_SHARED_EXEC
>
> -#define VMALLOC_SIZE     (KERN_VIRT_SIZE >> 1)
> -#define VMALLOC_END      (PAGE_OFFSET - 1)
> -#define VMALLOC_START    (PAGE_OFFSET - VMALLOC_SIZE)
> -
>  /*
>   * Roughly size the vmemmap space to be large enough to fit enough
>   * struct pages to map half the virtual address space. Then
> @@ -100,21 +96,6 @@ extern pgd_t swapper_pg_dir[];
>
>  #define vmemmap                ((struct page *)VMEMMAP_START)
>
> -#define FIXADDR_TOP      (VMEMMAP_START)
> -#ifdef CONFIG_64BIT
> -#define FIXADDR_SIZE     PMD_SIZE
> -#else
> -#define FIXADDR_SIZE     PGDIR_SIZE
> -#endif
> -#define FIXADDR_START    (FIXADDR_TOP - FIXADDR_SIZE)
> -
> -/*
> - * 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));
> @@ -428,13 +409,17 @@ 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 })
>
> -#ifdef CONFIG_FLATMEM
> -#define kern_addr_valid(addr)   (1) /* FIXME */
> -#endif
> +#define VMALLOC_SIZE           (KERN_VIRT_SIZE >> 1)
> +#define VMALLOC_END            (PAGE_OFFSET - 1)
> +#define VMALLOC_START          (PAGE_OFFSET - VMALLOC_SIZE)
>
> -extern void *dtb_early_va;
> -extern void setup_bootmem(void);
> -extern void paging_init(void);
> +#define FIXADDR_TOP      VMEMMAP_START
> +#ifdef CONFIG_64BIT
> +#define FIXADDR_SIZE     PMD_SIZE
> +#else
> +#define FIXADDR_SIZE     PGDIR_SIZE
> +#endif
> +#define FIXADDR_START    (FIXADDR_TOP - FIXADDR_SIZE)
>
>  /*
>   * Task size is 0x4000000000 for RV64 or 0x9fc00000 for RV32.
> @@ -446,6 +431,33 @@ extern void paging_init(void);
>  #define TASK_SIZE FIXADDR_START
>  #endif
>
> +#else /* CONFIG_MMU */
> +
> +#define PAGE_KERNEL            __pgprot(0)
> +#define swapper_pg_dir         NULL
> +#define VMALLOC_START          0
> +
> +#define TASK_SIZE 0xffffffffUL
> +
> +#endif /* !CONFIG_MMU */
> +
> +#ifdef CONFIG_FLATMEM
> +#define kern_addr_valid(addr)   (1) /* FIXME */
> +#endif
> +
> +extern void *dtb_early_va;
> +void setup_bootmem(void);
> +void paging_init(void);
> +
> +#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>
>
>  #endif /* !__ASSEMBLY__ */
> diff --git a/arch/riscv/include/asm/tlbflush.h b/arch/riscv/include/asm/tlbflush.h
> index f02188a5b0f4..394cfbccdcd9 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
>  static inline void local_flush_tlb_all(void)
>  {
>         __asm__ __volatile__ ("sfence.vma" : : : "memory");
> @@ -20,14 +21,19 @@ 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 */
>
> -#ifdef CONFIG_SMP
> +#if defined(CONFIG_SMP) && defined(CONFIG_MMU)
>  void flush_tlb_all(void);
>  void flush_tlb_mm(struct mm_struct *mm);
>  void flush_tlb_page(struct vm_area_struct *vma, unsigned long addr);
>  void flush_tlb_range(struct vm_area_struct *vma, unsigned long start,
>                      unsigned long end);
> -#else /* CONFIG_SMP */
> +#else /* CONFIG_SMP && CONFIG_MMU */
> +
>  #define flush_tlb_all() local_flush_tlb_all()
>  #define flush_tlb_page(vma, addr) local_flush_tlb_page(addr)
>
> @@ -38,7 +44,7 @@ static inline void flush_tlb_range(struct vm_area_struct *vma,
>  }
>
>  #define flush_tlb_mm(mm) flush_tlb_all()
> -#endif /* CONFIG_SMP */
> +#endif /* !CONFIG_SMP || !CONFIG_MMU */
>
>  /* Flush a range of kernel pages */
>  static inline void flush_tlb_kernel_range(unsigned long start,
> 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 2dca51046899..f40205cb9a22 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_RISCV_M_MODE)     += clint.o
>  obj-$(CONFIG_FPU)              += fpu.o
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index c0b3732af1ea..4bf6577dde7d 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -392,6 +392,10 @@ 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)
> @@ -413,3 +417,10 @@ 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 25867b99cc95..71efbba25ed5 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -109,8 +109,10 @@ clear_bss_done:
>         la sp, init_thread_union + THREAD_SIZE
>         mv a0, s1
>         call setup_vm
> +#ifdef CONFIG_MMU
>         la a0, early_pg_dir
>         call relocate
> +#endif /* CONFIG_MMU */
>
>         /* Restore C environment */
>         la tp, init_task
> @@ -121,6 +123,7 @@ clear_bss_done:
>         call parse_dtb
>         tail start_kernel
>
> +#ifdef CONFIG_MMU
>  relocate:
>         /* Relocate return address */
>         li a1, PAGE_OFFSET
> @@ -171,6 +174,7 @@ relocate:
>         sfence.vma
>
>         ret
> +#endif /* CONFIG_MMU */
>
>  .Lsecondary_start:
>  #ifdef CONFIG_SMP
> @@ -196,9 +200,11 @@ relocate:
>         beqz tp, .Lwait_for_cpu_up
>         fence
>
> +#ifdef CONFIG_MMU
>         /* Enable virtual memory and relocate to virtual address */
>         la a0, swapper_pg_dir
>         call relocate
> +#endif
>
>         tail smp_callin
>  #endif
> diff --git a/arch/riscv/kernel/signal.c b/arch/riscv/kernel/signal.c
> index e1a2cee340f7..eb8ba201feff 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 267feaa10f6a..47e7a8204460 100644
> --- a/arch/riscv/lib/Makefile
> +++ b/arch/riscv/lib/Makefile
> @@ -1,7 +1,6 @@
>  # SPDX-License-Identifier: GPL-2.0-only
> -lib-y  += delay.o
> -lib-y  += memcpy.o
> -lib-y  += memset.o
> -lib-y  += uaccess.o
> -
> -lib-$(CONFIG_64BIT) += tishift.o
> +lib-y                  += delay.o
> +lib-y                  += memcpy.o
> +lib-y                  += memset.o
> +lib-$(CONFIG_MMU)      += uaccess.o
> +lib-$(CONFIG_64BIT)    += tishift.o
> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
> index 9d9a17335686..44ab8f28c3fa 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 794c9ab256eb..8f1900686640 100644
> --- a/arch/riscv/mm/cacheflush.c
> +++ b/arch/riscv/mm/cacheflush.c
> @@ -78,6 +78,7 @@ 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);
> @@ -85,3 +86,4 @@ 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 83f7d12042fb..0b063f6acaa1 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -24,6 +24,7 @@ unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
>  EXPORT_SYMBOL(empty_zero_page);
>
>  extern char _start[];
> +void *dtb_early_va;
>
>  static void __init zone_sizes_init(void)
>  {
> @@ -140,12 +141,12 @@ void __init setup_bootmem(void)
>         }
>  }
>
> +#ifdef CONFIG_MMU
>  unsigned long va_pa_offset;
>  EXPORT_SYMBOL(va_pa_offset);
>  unsigned long pfn_base;
>  EXPORT_SYMBOL(pfn_base);
>
> -void *dtb_early_va;
>  pgd_t swapper_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>  pgd_t trampoline_pg_dir[PTRS_PER_PGD] __page_aligned_bss;
>  pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
> @@ -448,6 +449,16 @@ static void __init setup_vm_final(void)
>         csr_write(CSR_SATP, PFN_DOWN(__pa(swapper_pg_dir)) | SATP_MODE);
>         local_flush_tlb_all();
>  }
> +#else
> +asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> +{
> +       dtb_early_va = (void *)dtb_pa;
> +}
> +
> +static inline void setup_vm_final(void)
> +{
> +}
> +#endif /* CONFIG_MMU */
>
>  void __init paging_init(void)
>  {
> --
> 2.20.1
>

LGTM.

Reviewed-by: Anup Patel <anup@brainfault.org>

Regards,
Anup