Re: [RFC PATCH 01/12] riscv: Move kernel mapping outside of linear mapping

[Alex Ghiti <alex@xxxxxxxx>]

Hi Anup,

Le 1/5/21 à 6:40 AM, Anup Patel a écrit :
> On Tue, Jan 5, 2021 at 1:29 AM Alexandre Ghiti <alex@xxxxxxxx> wrote:
> > This is a preparatory patch for relocatable kernel and sv48 support.
> >
> > The kernel used to be linked at PAGE_OFFSET address therefore we could use
> > the linear mapping for the kernel mapping. But the relocated kernel base
> > address will be different from PAGE_OFFSET and since in the linear mapping,
> > two different virtual addresses cannot point to the same physical address,
> > the kernel mapping needs to lie outside the linear mapping so that we don't
> > have to copy it at the same physical offset.
> >
> > The kernel mapping is moved to the last 2GB of the address space and then
> > BPF and modules are also pushed to the same range since they have to lie
> > close to the kernel inside a 2GB window.
> >
> > Note then that KASLR implementation will simply have to move the kernel in
> > this 2GB range and modify BPF/modules regions accordingly.
> >
> > In addition, by moving the kernel to the end of the address space, both
> > sv39 and sv48 kernels will be exactly the same without needing to be
> > relocated at runtime.
>
> Awesome ! This is a good approach with no performance impact.
>
> > Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
> > Signed-off-by: Alexandre Ghiti <alex@xxxxxxxx>
> > ---
> >   arch/riscv/boot/loader.lds.S     |  3 +-
> >   arch/riscv/include/asm/page.h    | 10 ++++-
> >   arch/riscv/include/asm/pgtable.h | 39 +++++++++++++------
> >   arch/riscv/kernel/head.S         |  3 +-
> >   arch/riscv/kernel/module.c       |  4 +-
> >   arch/riscv/kernel/vmlinux.lds.S  |  3 +-
> >   arch/riscv/mm/init.c             | 65 ++++++++++++++++++++++++--------
> >   arch/riscv/mm/physaddr.c         |  2 +-
> >   8 files changed, 94 insertions(+), 35 deletions(-)
> >
> > diff --git a/arch/riscv/boot/loader.lds.S b/arch/riscv/boot/loader.lds.S
> > index 47a5003c2e28..62d94696a19c 100644
> > --- a/arch/riscv/boot/loader.lds.S
> > +++ b/arch/riscv/boot/loader.lds.S
> > @@ -1,13 +1,14 @@
> >   /* SPDX-License-Identifier: GPL-2.0 */
> >
> >   #include <asm/page.h>
> > +#include <asm/pgtable.h>
> >
> >   OUTPUT_ARCH(riscv)
> >   ENTRY(_start)
> >
> >   SECTIONS
> >   {
> > -       . = PAGE_OFFSET;
> > +       . = KERNEL_LINK_ADDR;
> >
> >          .payload : {
> >                  *(.payload)
> > diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> > index 2d50f76efe48..98188e315e8d 100644
> > --- a/arch/riscv/include/asm/page.h
> > +++ b/arch/riscv/include/asm/page.h
> > @@ -90,18 +90,26 @@ typedef struct page *pgtable_t;
> >
> >   #ifdef CONFIG_MMU
> >   extern unsigned long va_pa_offset;
> > +extern unsigned long va_kernel_pa_offset;
> >   extern unsigned long pfn_base;
> >   #define ARCH_PFN_OFFSET                (pfn_base)
> >   #else
> >   #define va_pa_offset           0
> > +#define va_kernel_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;
> > +extern unsigned long kernel_virt_addr;
> >
> >   #define __pa_to_va_nodebug(x)  ((void *)((unsigned long) (x) + va_pa_offset))
> > -#define __va_to_pa_nodebug(x)  ((unsigned long)(x) - va_pa_offset)
> > +#define linear_mapping_va_to_pa(x)     ((unsigned long)(x) - va_pa_offset)
> > +#define kernel_mapping_va_to_pa(x)     \
> > +       ((unsigned long)(x) - va_kernel_pa_offset)
> > +#define __va_to_pa_nodebug(x)          \
> > +       (((x) < KERNEL_LINK_ADDR) ?             \
> > +               linear_mapping_va_to_pa(x) : kernel_mapping_va_to_pa(x))
> >
> >   #ifdef CONFIG_DEBUG_VIRTUAL
> >   extern phys_addr_t __virt_to_phys(unsigned long x);
> > diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> > index 183f1f4b2ae6..102b728ca146 100644
> > --- a/arch/riscv/include/asm/pgtable.h
> > +++ b/arch/riscv/include/asm/pgtable.h
> > @@ -11,23 +11,32 @@
> >
> >   #include <asm/pgtable-bits.h>
> >
> > -#ifndef __ASSEMBLY__
> > -
> > -/* Page Upper Directory not used in RISC-V */
> > -#include <asm-generic/pgtable-nopud.h>
> > -#include <asm/page.h>
> > -#include <asm/tlbflush.h>
> > -#include <linux/mm_types.h>
> > +#ifndef CONFIG_MMU
> > +#define KERNEL_VIRT_ADDR       PAGE_OFFSET
> > +#define KERNEL_LINK_ADDR       PAGE_OFFSET
> > +#else
> >
> > -#ifdef CONFIG_MMU
> > +#define ADDRESS_SPACE_END      (UL(-1))
> > +/*
> > + * Leave 2GB for kernel, modules and BPF at the end of the address space
> > + */
> > +#define KERNEL_VIRT_ADDR       (ADDRESS_SPACE_END - SZ_2G + 1)
> > +#define KERNEL_LINK_ADDR       KERNEL_VIRT_ADDR
> >
> >   #define VMALLOC_SIZE     (KERN_VIRT_SIZE >> 1)
> >   #define VMALLOC_END      (PAGE_OFFSET - 1)
> >   #define VMALLOC_START    (PAGE_OFFSET - VMALLOC_SIZE)
> >
> > +/* KASLR should leave at least 128MB for BPF after the kernel */
> >   #define BPF_JIT_REGION_SIZE    (SZ_128M)
> > -#define BPF_JIT_REGION_START   (PAGE_OFFSET - BPF_JIT_REGION_SIZE)
> > -#define BPF_JIT_REGION_END     (VMALLOC_END)
> > +#define BPF_JIT_REGION_START   PFN_ALIGN((unsigned long)&_end)
> > +#define BPF_JIT_REGION_END     (BPF_JIT_REGION_START + BPF_JIT_REGION_SIZE)
> > +
> > +/* Modules always live before the kernel */
> > +#ifdef CONFIG_64BIT
> > +#define VMALLOC_MODULE_START   (PFN_ALIGN((unsigned long)&_end) - SZ_2G)
> > +#define VMALLOC_MODULE_END     (PFN_ALIGN((unsigned long)&_start))
> > +#endif
>
> This does not look right or I am missing something.
>
> I think the VMALLOC_MODULE_START should be:
> #define VMALLOC_MODULE_START   (PFN_ALIGN((unsigned long)&_start) - SZ_2G)

I think the patch is correct: worst-case, we want the first address of 
the module area to be able to access the last address of the kernel, so 
we must use _end and not _start to guarantee that the difference between 
those 2 addresses is not greater than 2GB.

> >   /*
> >    * Roughly size the vmemmap space to be large enough to fit enough
> > @@ -57,9 +66,16 @@
> >   #define FIXADDR_SIZE     PGDIR_SIZE
> >   #endif
> >   #define FIXADDR_START    (FIXADDR_TOP - FIXADDR_SIZE)
> > -
> >   #endif
> >
> > +#ifndef __ASSEMBLY__
> > +
> > +/* Page Upper Directory not used in RISC-V */
> > +#include <asm-generic/pgtable-nopud.h>
> > +#include <asm/page.h>
> > +#include <asm/tlbflush.h>
> > +#include <linux/mm_types.h>
> > +
> >   #ifdef CONFIG_64BIT
> >   #include <asm/pgtable-64.h>
> >   #else
> > @@ -467,6 +483,7 @@ static inline void __kernel_map_pages(struct page *page, int numpages, int enabl
> >
> >   #define kern_addr_valid(addr)   (1) /* FIXME */
> >
> > +extern char _start[];
> >   extern void *dtb_early_va;
> >   extern uintptr_t dtb_early_pa;
> >   void setup_bootmem(void);
> > diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> > index 7e849797c9c3..66f40c49bf68 100644
> > --- a/arch/riscv/kernel/head.S
> > +++ b/arch/riscv/kernel/head.S
> > @@ -69,7 +69,8 @@ pe_head_start:
> >   #ifdef CONFIG_MMU
> >   relocate:
> >          /* Relocate return address */
> > -       li a1, PAGE_OFFSET
> > +       la a1, kernel_virt_addr
> > +       REG_L a1, 0(a1)
> >          la a2, _start
> >          sub a1, a1, a2
> >          add ra, ra, a1
> > diff --git a/arch/riscv/kernel/module.c b/arch/riscv/kernel/module.c
> > index 104fba889cf7..75a0b9541266 100644
> > --- a/arch/riscv/kernel/module.c
> > +++ b/arch/riscv/kernel/module.c
> > @@ -408,12 +408,10 @@ int apply_relocate_add(Elf_Shdr *sechdrs, const char *strtab,
> >   }
> >
> >   #if defined(CONFIG_MMU) && defined(CONFIG_64BIT)
> > -#define VMALLOC_MODULE_START \
> > -        max(PFN_ALIGN((unsigned long)&_end - SZ_2G), VMALLOC_START)
> >   void *module_alloc(unsigned long size)
> >   {
> >          return __vmalloc_node_range(size, 1, VMALLOC_MODULE_START,
> > -                                   VMALLOC_END, GFP_KERNEL,
> > +                                   VMALLOC_MODULE_END, GFP_KERNEL,
> >                                      PAGE_KERNEL_EXEC, 0, NUMA_NO_NODE,
> >                                      __builtin_return_address(0));
> >   }
> > diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
> > index 3ffbd6cbdb86..c21dc46f41be 100644
> > --- a/arch/riscv/kernel/vmlinux.lds.S
> > +++ b/arch/riscv/kernel/vmlinux.lds.S
> > @@ -4,7 +4,8 @@
> >    * Copyright (C) 2017 SiFive
> >    */
> >
> > -#define LOAD_OFFSET PAGE_OFFSET
> > +#include <asm/pgtable.h>
> > +#define LOAD_OFFSET KERNEL_LINK_ADDR
> >   #include <asm/vmlinux.lds.h>
> >   #include <asm/page.h>
> >   #include <asm/cache.h>
> > diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> > index 8e577f14f120..9d06ff0e015a 100644
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -23,6 +23,9 @@
> >
> >   #include "../kernel/head.h"
> >
> > +unsigned long kernel_virt_addr = KERNEL_VIRT_ADDR;
> > +EXPORT_SYMBOL(kernel_virt_addr);
> > +
> >   unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]
> >                                                          __page_aligned_bss;
> >   EXPORT_SYMBOL(empty_zero_page);
> > @@ -201,8 +204,12 @@ void __init setup_bootmem(void)
> >   #ifdef CONFIG_MMU
> >   static struct pt_alloc_ops pt_ops;
> >
> > +/* Offset between linear mapping virtual address and kernel load address */
> >   unsigned long va_pa_offset;
> >   EXPORT_SYMBOL(va_pa_offset);
> > +/* Offset between kernel mapping virtual address and kernel load address */
> > +unsigned long va_kernel_pa_offset;
> > +EXPORT_SYMBOL(va_kernel_pa_offset);
> >   unsigned long pfn_base;
> >   EXPORT_SYMBOL(pfn_base);
> >
> > @@ -316,7 +323,7 @@ static phys_addr_t __init alloc_pmd_early(uintptr_t va)
> >   {
> >          uintptr_t pmd_num;
> >
> > -       pmd_num = (va - PAGE_OFFSET) >> PGDIR_SHIFT;
> > +       pmd_num = (va - kernel_virt_addr) >> PGDIR_SHIFT;
> >          BUG_ON(pmd_num >= NUM_EARLY_PMDS);
> >          return (uintptr_t)&early_pmd[pmd_num * PTRS_PER_PMD];
> >   }
> > @@ -431,17 +438,34 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
> >   #error "setup_vm() is called from head.S before relocate so it should not use absolute addressing."
> >   #endif
> >
> > +static uintptr_t load_pa, load_sz;
> > +
> > +static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
> > +{
> > +       uintptr_t va, end_va;
> > +
> > +       end_va = kernel_virt_addr + load_sz;
> > +       for (va = kernel_virt_addr; va < end_va; va += map_size)
> > +               create_pgd_mapping(pgdir, va,
> > +                                  load_pa + (va - kernel_virt_addr),
> > +                                  map_size, PAGE_KERNEL_EXEC);
> > +}
> > +
> >   asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> >   {
> > -       uintptr_t va, pa, end_va;
> > -       uintptr_t load_pa = (uintptr_t)(&_start);
> > -       uintptr_t load_sz = (uintptr_t)(&_end) - load_pa;
> > -       uintptr_t map_size = best_map_size(load_pa, MAX_EARLY_MAPPING_SIZE);
> > +       uintptr_t pa;
> > +       uintptr_t map_size;
> >   #ifndef __PAGETABLE_PMD_FOLDED
> >          pmd_t fix_bmap_spmd, fix_bmap_epmd;
> >   #endif
> >
> > +       load_pa = (uintptr_t)(&_start);
> > +       load_sz = (uintptr_t)(&_end) - load_pa;
> > +       map_size = best_map_size(load_pa, MAX_EARLY_MAPPING_SIZE);
> > +
> >          va_pa_offset = PAGE_OFFSET - load_pa;
> > +       va_kernel_pa_offset = kernel_virt_addr - load_pa;
> > +
> >          pfn_base = PFN_DOWN(load_pa);
> >
> >          /*
> > @@ -470,26 +494,22 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
> >          create_pmd_mapping(fixmap_pmd, FIXADDR_START,
> >                             (uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
> >          /* Setup trampoline PGD and PMD */
> > -       create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
> > +       create_pgd_mapping(trampoline_pg_dir, kernel_virt_addr,
> >                             (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
> > -       create_pmd_mapping(trampoline_pmd, PAGE_OFFSET,
> > +       create_pmd_mapping(trampoline_pmd, kernel_virt_addr,
> >                             load_pa, PMD_SIZE, PAGE_KERNEL_EXEC);
> >   #else
> >          /* Setup trampoline PGD */
> > -       create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
> > +       create_pgd_mapping(trampoline_pg_dir, kernel_virt_addr,
> >                             load_pa, PGDIR_SIZE, PAGE_KERNEL_EXEC);
> >   #endif
> >
> >          /*
> > -        * Setup early PGD covering entire kernel which will allows
> > +        * Setup early PGD covering entire kernel which will allow
> >           * us to reach paging_init(). We map all memory banks later
> >           * in setup_vm_final() below.
> >           */
> > -       end_va = PAGE_OFFSET + load_sz;
> > -       for (va = PAGE_OFFSET; va < end_va; va += map_size)
> > -               create_pgd_mapping(early_pg_dir, va,
> > -                                  load_pa + (va - PAGE_OFFSET),
> > -                                  map_size, PAGE_KERNEL_EXEC);
> > +       create_kernel_page_table(early_pg_dir, map_size);
> >
> >   #ifndef __PAGETABLE_PMD_FOLDED
> >          /* Setup early PMD for DTB */
> > @@ -549,6 +569,7 @@ static void __init setup_vm_final(void)
> >          uintptr_t va, map_size;
> >          phys_addr_t pa, start, end;
> >          u64 i;
> > +       static struct vm_struct vm_kernel = { 0 };
> >
> >          /**
> >           * MMU is enabled at this point. But page table setup is not complete yet.
> > @@ -565,7 +586,7 @@ static void __init setup_vm_final(void)
> >                             __pa_symbol(fixmap_pgd_next),
> >                             PGDIR_SIZE, PAGE_TABLE);
> >
> > -       /* Map all memory banks */
> > +       /* Map all memory banks in the linear mapping */
> >          for_each_mem_range(i, &start, &end) {
> >                  if (start >= end)
> >                          break;
> > @@ -577,10 +598,22 @@ static void __init setup_vm_final(void)
> >                  for (pa = start; pa < end; pa += map_size) {
> >                          va = (uintptr_t)__va(pa);
> >                          create_pgd_mapping(swapper_pg_dir, va, pa,
> > -                                          map_size, PAGE_KERNEL_EXEC);
> > +                                          map_size, PAGE_KERNEL);
> >                  }
> >          }
> >
> > +       /* Map the kernel */
> > +       create_kernel_page_table(swapper_pg_dir, PMD_SIZE);
> > +
> > +       /* Reserve the vmalloc area occupied by the kernel */
> > +       vm_kernel.addr = (void *)kernel_virt_addr;
> > +       vm_kernel.phys_addr = load_pa;
> > +       vm_kernel.size = (load_sz + PMD_SIZE - 1) & ~(PMD_SIZE - 1);
> > +       vm_kernel.flags = VM_MAP | VM_NO_GUARD;
> > +       vm_kernel.caller = __builtin_return_address(0);
> > +
> > +       vm_area_add_early(&vm_kernel);
> > +
> >          /* Clear fixmap PTE and PMD mappings */
> >          clear_fixmap(FIX_PTE);
> >          clear_fixmap(FIX_PMD);
> > diff --git a/arch/riscv/mm/physaddr.c b/arch/riscv/mm/physaddr.c
> > index e8e4dcd39fed..35703d5ef5fd 100644
> > --- a/arch/riscv/mm/physaddr.c
> > +++ b/arch/riscv/mm/physaddr.c
> > @@ -23,7 +23,7 @@ EXPORT_SYMBOL(__virt_to_phys);
> >
> >   phys_addr_t __phys_addr_symbol(unsigned long x)
> >   {
> > -       unsigned long kernel_start = (unsigned long)PAGE_OFFSET;
> > +       unsigned long kernel_start = (unsigned long)kernel_virt_addr;
> >          unsigned long kernel_end = (unsigned long)_end;
> >
> >          /*
> > --
> > 2.20.1
>
> Apart from the minor comment above, this looks good to me.
>
> Reviewed-by: Anup Patel <anup@xxxxxxxxxxxxxx>

Thanks for your time Anup,

> Regards,
> Anup

Alex