Re: [PATCH RFC] riscv: Remove all XIP fixups by initializing MMU in memory

From: Jisheng Zhang
Date: Fri Jun 18 2021 - 12:30:14 EST

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In reply to: Alexandre Ghiti: "[PATCH RFC] riscv: Remove all XIP fixups by initializing MMU in memory"
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On Fri, 18 Jun 2021 16:23:53 +0200
Alexandre Ghiti <alex@xxxxxxxx> wrote:

> For XIP kernel, the variables that are accessed when the MMU is off must be
> fixup and it is currently achieved by adding a preprocessor macro for all
> those variables. This comes with a few drawbacks:
>
> - it adds a lot of code, especially in mm/init.c which gets overloaded,
> - it easily breaks XIP kernels as anyone accessing a global variable before
> the MMU is on must fixup this variable,
> - it prevents the usage of those fixup variables anywhere outside
> mm/init.c: kernel addresses conversion macros would benefit to switch
> to inline functions.
>
> This patch makes the data fixups unnecessary by initializing the MMU in the
> memory rather than in flash. Indeed, very soon in the boot process, the
> kernel is copied to memory and the execution continues from there until
> the kernel mapping is established where the execution jumps back in
> flash. Then the offsets from the PC are preserved and no fixup is
> necessary. And the kernel text in memory is never reserved so it is free
> to use later on.

To be honest, the XIP enabling patch makes the code complex. This patch cleans up
the code a lot! Thank you so much.

>
> This solution has the following drawbacks:
>
> - XIP kernel boot is longer because of the copy to memory, but can be
> improved by copying only necessary parts,
> - in the current implementation, it creates a hole at the beginning of
> the memory which could prevent large contiguous allocation,
> - it actually just inverts what needs to be fixup: now rodata and text
> symbols accessed during the initialization of the MMU must be fixup
> whereas it was data before. However, I found a single fixup to be
> necessary and it is the dtb physical address in case CONFIG_BUILTIN_DTB
> is set.
>
> Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> ---
> arch/riscv/include/asm/page.h | 12 ++---
> arch/riscv/include/asm/pgtable.h | 22 +-------
> arch/riscv/kernel/head.S | 87 +++++++++++++++++++++++---------
> arch/riscv/kernel/setup.c | 2 +-
> arch/riscv/mm/init.c | 59 +++-------------------
> 5 files changed, 78 insertions(+), 104 deletions(-)
>
> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> index 5d4622a44b09..97a950bbd3bf 100644
> --- a/arch/riscv/include/asm/page.h
> +++ b/arch/riscv/include/asm/page.h
> @@ -105,11 +105,11 @@ extern unsigned long kernel_virt_addr;
> ((x) >= PAGE_OFFSET && (x) < kernel_virt_addr)
>
> #define linear_mapping_pa_to_va(x) ((void *)((unsigned long)(x) + va_pa_offset))
> -#define kernel_mapping_pa_to_va(y) ({ \
> - unsigned long _y = y; \
> - (_y >= CONFIG_PHYS_RAM_BASE) ? \
> - (void *)((unsigned long)(_y) + va_kernel_pa_offset + XIP_OFFSET) : \
> - (void *)((unsigned long)(_y) + va_kernel_xip_pa_offset); \
> +#define kernel_mapping_pa_to_va(y) ({ \
> + unsigned long _y = y; \
> + (_y >= CONFIG_PHYS_RAM_BASE) ? \
> + (void *)((unsigned long)(_y) + va_kernel_pa_offset) : \
> + (void *)((unsigned long)(_y) + va_kernel_xip_pa_offset); \
> })
> #define __pa_to_va_nodebug(x) linear_mapping_pa_to_va(x)
>
> @@ -118,7 +118,7 @@ extern unsigned long kernel_virt_addr;
> unsigned long _y = y; \
> (_y < kernel_virt_addr + XIP_OFFSET) ? \
> ((unsigned long)(_y) - va_kernel_xip_pa_offset) : \
> - ((unsigned long)(_y) - va_kernel_pa_offset - XIP_OFFSET); \
> + ((unsigned long)(_y) - va_kernel_pa_offset); \
> })
>
> #define __va_to_pa_nodebug(x) ({ \
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 3ccd2dc52e85..99ec99384bf0 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -95,17 +95,6 @@
> #include <asm/pgtable-32.h>
> #endif /* CONFIG_64BIT */
>
> -#ifdef CONFIG_XIP_KERNEL
> -#define XIP_FIXUP(addr) ({ \
> - uintptr_t __a = (uintptr_t)(addr); \
> - (__a >= CONFIG_XIP_PHYS_ADDR && __a < CONFIG_XIP_PHYS_ADDR + SZ_16M) ? \
> - __a - CONFIG_XIP_PHYS_ADDR + CONFIG_PHYS_RAM_BASE - XIP_OFFSET :\
> - __a; \
> - })
> -#else
> -#define XIP_FIXUP(addr) (addr)
> -#endif /* CONFIG_XIP_KERNEL */
> -
> #ifdef CONFIG_MMU
> /* Number of entries in the page global directory */
> #define PTRS_PER_PGD (PAGE_SIZE / sizeof(pgd_t))
> @@ -683,15 +672,8 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
> #define kern_addr_valid(addr) (1) /* FIXME */
>
> extern char _start[];
> -extern void *_dtb_early_va;
> -extern uintptr_t _dtb_early_pa;
> -#if defined(CONFIG_XIP_KERNEL) && defined(CONFIG_MMU)
> -#define dtb_early_va (*(void **)XIP_FIXUP(&_dtb_early_va))
> -#define dtb_early_pa (*(uintptr_t *)XIP_FIXUP(&_dtb_early_pa))
> -#else
> -#define dtb_early_va _dtb_early_va
> -#define dtb_early_pa _dtb_early_pa
> -#endif /* CONFIG_XIP_KERNEL */
> +extern void *dtb_early_va;
> +extern uintptr_t dtb_early_pa;
>
> void paging_init(void);
> void misc_mem_init(void);
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 89cc58ab52b4..7ac7d2bd1a53 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -17,14 +17,33 @@
>
> #ifdef CONFIG_XIP_KERNEL
> .macro XIP_FIXUP_OFFSET reg
> - REG_L t0, _xip_fixup
> + li t0, CONFIG_PHYS_RAM_BASE - CONFIG_XIP_PHYS_ADDR
> add \reg, \reg, t0
> .endm
> -_xip_fixup: .dword CONFIG_PHYS_RAM_BASE - CONFIG_XIP_PHYS_ADDR - XIP_OFFSET
> +
> +.macro MEM_FIXUP_OFFSET reg
> + li t0, CONFIG_XIP_PHYS_ADDR - CONFIG_PHYS_RAM_BASE
> + add \reg, \reg, t0
> +.endm
> +
> +.macro XIP_JUMP_TO_MEM
> + la t1, 0f
> + XIP_FIXUP_OFFSET t1
> + jalr ra, t1, 0
> +0:
> + /* Reload the global pointer: we are now in memory! */
> +.option push
> +.option norelax
> + la gp, __global_pointer$
> +.option pop
> +.endm
> #else
> -.macro XIP_FIXUP_OFFSET reg
> +.macro MEM_FIXUP_OFFSET reg
> +.endm
> +
> +.macro XIP_JUMP_TO_MEM
> .endm
> -#endif /* CONFIG_XIP_KERNEL */
> +#endif
>
> __HEAD
> ENTRY(_start)
> @@ -82,7 +101,6 @@ pe_head_start:
> relocate:
> /* Relocate return address */
> la a1, kernel_virt_addr
> - XIP_FIXUP_OFFSET a1
> REG_L a1, 0(a1)
> la a2, _start
> sub a1, a1, a2
> @@ -105,7 +123,6 @@ relocate:
> * to ensure the new translations are in use.
> */
> la a0, trampoline_pg_dir
> - XIP_FIXUP_OFFSET a0
> srl a0, a0, PAGE_SHIFT
> or a0, a0, a1
> sfence.vma
> @@ -159,9 +176,7 @@ secondary_start_sbi:
>
> slli a3, a0, LGREG
> la a4, __cpu_up_stack_pointer
> - XIP_FIXUP_OFFSET a4
> la a5, __cpu_up_task_pointer
> - XIP_FIXUP_OFFSET a5
> add a4, a3, a4
> add a5, a3, a5
> REG_L sp, (a4)
> @@ -173,7 +188,6 @@ secondary_start_common:
> #ifdef CONFIG_MMU
> /* Enable virtual memory and relocate to virtual address */
> la a0, swapper_pg_dir
> - XIP_FIXUP_OFFSET a0
> call relocate
> #endif
> call setup_trap_vector
> @@ -253,14 +267,12 @@ pmp_done:
> tail .Lsecondary_park
> .Lgood_cores:
> #endif
> -
> #ifndef CONFIG_XIP_KERNEL
> /* Pick one hart to run the main boot sequence */
> la a3, hart_lottery
> li a2, 1
> amoadd.w a3, a2, (a3)
> bnez a3, .Lsecondary_start
> -
> #else
> /* hart_lottery in flash contains a magic number */
> la a3, hart_lottery
> @@ -270,17 +282,43 @@ pmp_done:
> amoswap.w t0, t1, (a2)
> /* first time here if hart_lottery in RAM is not set */
> beq t0, t1, .Lsecondary_start
> -
> - la sp, _end + THREAD_SIZE
> - XIP_FIXUP_OFFSET sp
> + /*
> + * Copy the kernel text and data to memory: the virtual mapping will be
> + * established from there, and then we will jump back to using flash
> + * resident text. This avoids to fixup global symbols when the code
> + * is executed from flash and targets data in memory before the MMU is
> + * enabled.
> + * We must preserve a0, a1 and we have no stack yet (__memcpy does not
> + * spill anything).
> + */
> mv s0, a0
> - call __copy_data
> + mv s1, a1
> +
> + li a0, CONFIG_PHYS_RAM_BASE
> + la a1, _xiprom
> + la a2, _exiprom
> + sub a2, a2, a1
> + add a2, a2, 1
> + call __memcpy
> +
> + li a0, CONFIG_PHYS_RAM_BASE + XIP_OFFSET
> + la a1, _sdata
> + la a2, _end
> + sub a2, a2, a1
> + add a2, a2, 1
> + call __memcpy
> +
> + fence.i
>
> - /* Restore a0 copy */
> mv a0, s0
> -#endif
> + mv a1, s1
>
> -#ifndef CONFIG_XIP_KERNEL
> + /*
> + * From here, the code will be executed from memory and we'll jump back
> + * to flash once the MMU is enabled.
> + */
> + XIP_JUMP_TO_MEM
> +#endif
> /* Clear BSS for flat non-ELF images */
> la a3, __bss_start
> la a4, __bss_stop
> @@ -290,27 +328,24 @@ clear_bss:
> add a3, a3, RISCV_SZPTR
> blt a3, a4, clear_bss
> clear_bss_done:
> -#endif
> /* Save hart ID and DTB physical address */
> mv s0, a0
> mv s1, a1
>
> la a2, boot_cpu_hartid
> - XIP_FIXUP_OFFSET a2
> REG_S a0, (a2)
>
> /* Initialize page tables and relocate to virtual addresses */
> la sp, init_thread_union + THREAD_SIZE
> - XIP_FIXUP_OFFSET sp
> #ifdef CONFIG_BUILTIN_DTB
> la a0, __dtb_start
> + MEM_FIXUP_OFFSET a0
> #else
> mv a0, s1
> #endif /* CONFIG_BUILTIN_DTB */
> call setup_vm
> #ifdef CONFIG_MMU
> la a0, early_pg_dir
> - XIP_FIXUP_OFFSET a0
> call relocate
> #endif /* CONFIG_MMU */
>
> @@ -329,15 +364,19 @@ clear_bss_done:
>
> .Lsecondary_start:
> #ifdef CONFIG_SMP
> + /*
> + * From here, XIP kernel will be executed from memory and we'll jump back
> + * to flash once the MMU is enabled.
> + */
> + XIP_JUMP_TO_MEM
> +
> /* Set trap vector to spin forever to help debug */
> la a3, .Lsecondary_park
> csrw CSR_TVEC, a3
>
> slli a3, a0, LGREG
> la a1, __cpu_up_stack_pointer
> - XIP_FIXUP_OFFSET a1
> la a2, __cpu_up_task_pointer
> - XIP_FIXUP_OFFSET a2
> add a1, a3, a1
> add a2, a3, a2
>
> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
> index 8b7f1c791821..8b7958b30a05 100644
> --- a/arch/riscv/kernel/setup.c
> +++ b/arch/riscv/kernel/setup.c
> @@ -279,7 +279,7 @@ void __init setup_arch(char **cmdline_p)
> #if IS_ENABLED(CONFIG_BUILTIN_DTB)
> unflatten_and_copy_device_tree();
> #else
> - if (early_init_dt_verify(__va(XIP_FIXUP(dtb_early_pa))))
> + if (early_init_dt_verify(__va(dtb_early_pa)))
> unflatten_device_tree();
> else
> pr_err("No DTB found in kernel mappings\n");
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 3d77b04bec54..a70cdd77b2be 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -33,7 +33,6 @@
> unsigned long kernel_virt_addr = KERNEL_LINK_ADDR;
> EXPORT_SYMBOL(kernel_virt_addr);
> #ifdef CONFIG_XIP_KERNEL
> -#define kernel_virt_addr (*((unsigned long *)XIP_FIXUP(&kernel_virt_addr)))
> extern char _xiprom[], _exiprom[];
> #endif
>
> @@ -43,8 +42,8 @@ EXPORT_SYMBOL(empty_zero_page);
>
> extern char _start[];
> #define DTB_EARLY_BASE_VA PGDIR_SIZE
> -void *_dtb_early_va __initdata;
> -uintptr_t _dtb_early_pa __initdata;
> +void *dtb_early_va __initdata;
> +uintptr_t dtb_early_pa __initdata;
>
> struct pt_alloc_ops {
> pte_t *(*get_pte_virt)(phys_addr_t pa);
> @@ -203,33 +202,18 @@ static void __init setup_bootmem(void)
> }
>
> #ifdef CONFIG_MMU
> -static struct pt_alloc_ops _pt_ops __initdata;
> -
> -#ifdef CONFIG_XIP_KERNEL
> -#define pt_ops (*(struct pt_alloc_ops *)XIP_FIXUP(&_pt_ops))
> -#else
> -#define pt_ops _pt_ops
> -#endif
> +static struct pt_alloc_ops pt_ops __initdata;
>
> /* Offset between linear mapping virtual address and kernel load address */
> unsigned long va_pa_offset __ro_after_init;
> EXPORT_SYMBOL(va_pa_offset);
> -#ifdef CONFIG_XIP_KERNEL
> -#define va_pa_offset (*((unsigned long *)XIP_FIXUP(&va_pa_offset)))
> -#endif
> /* Offset between kernel mapping virtual address and kernel load address */
> #ifdef CONFIG_64BIT
> unsigned long va_kernel_pa_offset __ro_after_init;
> EXPORT_SYMBOL(va_kernel_pa_offset);
> #endif
> -#ifdef CONFIG_XIP_KERNEL
> -#define va_kernel_pa_offset (*((unsigned long *)XIP_FIXUP(&va_kernel_pa_offset)))
> -#endif
> unsigned long va_kernel_xip_pa_offset __ro_after_init;
> EXPORT_SYMBOL(va_kernel_xip_pa_offset);
> -#ifdef CONFIG_XIP_KERNEL
> -#define va_kernel_xip_pa_offset (*((unsigned long *)XIP_FIXUP(&va_kernel_xip_pa_offset)))
> -#endif
> unsigned long pfn_base __ro_after_init;
> EXPORT_SYMBOL(pfn_base);
>
> @@ -239,12 +223,6 @@ static pte_t fixmap_pte[PTRS_PER_PTE] __page_aligned_bss;
>
> pgd_t early_pg_dir[PTRS_PER_PGD] __initdata __aligned(PAGE_SIZE);
>
> -#ifdef CONFIG_XIP_KERNEL
> -#define trampoline_pg_dir ((pgd_t *)XIP_FIXUP(trampoline_pg_dir))
> -#define fixmap_pte ((pte_t *)XIP_FIXUP(fixmap_pte))
> -#define early_pg_dir ((pgd_t *)XIP_FIXUP(early_pg_dir))
> -#endif /* CONFIG_XIP_KERNEL */
> -
> void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
> {
> unsigned long addr = __fix_to_virt(idx);
> @@ -320,12 +298,6 @@ static pmd_t fixmap_pmd[PTRS_PER_PMD] __page_aligned_bss;
> static pmd_t early_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
> static pmd_t early_dtb_pmd[PTRS_PER_PMD] __initdata __aligned(PAGE_SIZE);
>
> -#ifdef CONFIG_XIP_KERNEL
> -#define trampoline_pmd ((pmd_t *)XIP_FIXUP(trampoline_pmd))
> -#define fixmap_pmd ((pmd_t *)XIP_FIXUP(fixmap_pmd))
> -#define early_pmd ((pmd_t *)XIP_FIXUP(early_pmd))
> -#endif /* CONFIG_XIP_KERNEL */
> -
> static pmd_t *__init get_pmd_virt_early(phys_addr_t pa)
> {
> /* Before MMU is enabled */
> @@ -442,19 +414,6 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
> return PMD_SIZE;
> }
>
> -#ifdef CONFIG_XIP_KERNEL
> -/* called from head.S with MMU off */
> -asmlinkage void __init __copy_data(void)
> -{
> - void *from = (void *)(&_sdata);
> - void *end = (void *)(&_end);
> - void *to = (void *)CONFIG_PHYS_RAM_BASE;
> - size_t sz = (size_t)(end - from + 1);
> -
> - memcpy(to, from, sz);
> -}
> -#endif
> -
> #ifdef CONFIG_STRICT_KERNEL_RWX
> static __init pgprot_t pgprot_from_va(uintptr_t va)
> {
> @@ -511,16 +470,10 @@ static __init pgprot_t pgprot_from_va(uintptr_t va)
>
> static uintptr_t load_pa __initdata;
> uintptr_t load_sz;
> -#ifdef CONFIG_XIP_KERNEL
> -#define load_pa (*((uintptr_t *)XIP_FIXUP(&load_pa)))
> -#define load_sz (*((uintptr_t *)XIP_FIXUP(&load_sz)))
> -#endif
>
> #ifdef CONFIG_XIP_KERNEL
> static uintptr_t xiprom __initdata;
> static uintptr_t xiprom_sz __initdata;
> -#define xiprom_sz (*((uintptr_t *)XIP_FIXUP(&xiprom_sz)))
> -#define xiprom (*((uintptr_t *)XIP_FIXUP(&xiprom)))
>
> static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size,
> __always_unused bool early)
> @@ -538,7 +491,7 @@ static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size,
> end_va = kernel_virt_addr + XIP_OFFSET + load_sz;
> for (va = kernel_virt_addr + XIP_OFFSET; va < end_va; va += map_size)
> create_pgd_mapping(pgdir, va,
> - load_pa + (va - (kernel_virt_addr + XIP_OFFSET)),
> + load_pa + (va - kernel_virt_addr),
> map_size, PAGE_KERNEL);
> }
> #else
> @@ -648,7 +601,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> * whereas dtb_early_va will be used before setup_vm_final installs
> * the linear mapping.
> */
> - dtb_early_va = kernel_mapping_pa_to_va(XIP_FIXUP(dtb_pa));
> + dtb_early_va = kernel_mapping_pa_to_va(dtb_pa);
> #else
> dtb_early_va = __va(dtb_pa);
> #endif /* CONFIG_64BIT */
> @@ -664,7 +617,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> dtb_early_va = (void *)DTB_EARLY_BASE_VA + (dtb_pa & (PGDIR_SIZE - 1));
> #else /* CONFIG_BUILTIN_DTB */
> #ifdef CONFIG_64BIT
> - dtb_early_va = kernel_mapping_pa_to_va(XIP_FIXUP(dtb_pa));
> + dtb_early_va = kernel_mapping_pa_to_va(dtb_pa);
> #else
> dtb_early_va = __va(dtb_pa);
> #endif /* CONFIG_64BIT */

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In reply to: Alexandre Ghiti: "[PATCH RFC] riscv: Remove all XIP fixups by initializing MMU in memory"
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