[PATCH] riscv: toggle mmu_enabled flag in a precise manner

[Atish Patra]

On Tue, Dec 1, 2020 at 1:01 AM <vitaly.wool at konsulko.com> wrote:
>
> From: Vitaly Wool <vitaly.wool at konsulko.com>
>
> Cuurently mmu_enabled flag is set to true way later than the actual
> MMU enablement takes place. This leads to hard-to-track races in
> e. g. SBI earlycon initialization taking wrong path configuring
> fixmap.
>

This code path is significantly changed in 5.10-rcX with UEFI series.
https://patchwork.kernel.org/project/linux-riscv/patch/20200917223716.2300238-4-atish.patra@wdc.com/

Can you check if you can still reproduce the issue you were seeing
with the latest upstream kernel ?
If yes, please share the steps to reproduce the issue.

> To fix that, move mmu_enabled toggling to head.S and rename it to
> _mmu_enabled to avoid possible name clashes since it's not a static
> variable any more.
>
> Signed-off-by: Vitaly Wool <vitaly.wool at konsulko.com>
> ---
>  arch/riscv/kernel/head.S |  9 +++++++++
>  arch/riscv/mm/init.c     | 13 +++++--------
>  2 files changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 0a4e81b8dc79..33cd57285be3 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -248,6 +248,10 @@ clear_bss_done:
>         call relocate
>  #endif /* CONFIG_MMU */
>
> +       la a0, _mmu_enabled
> +       li a1, 1
> +       sw a1, (a0)
> +
>         call setup_trap_vector
>         /* Restore C environment */
>         la tp, init_task
> @@ -370,6 +374,11 @@ ENTRY(reset_regs)
>  END(reset_regs)
>  #endif /* CONFIG_RISCV_M_MODE */
>
> +.section ".data"
> +       .global _mmu_enabled
> +_mmu_enabled:
> +       .word 0
> +
>  __PAGE_ALIGNED_BSS
>         /* Empty zero page */
>         .balign PAGE_SIZE
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 787c75f751a5..4038be635e25 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -211,7 +211,7 @@ EXPORT_SYMBOL(pfn_base);
>  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;
> -static bool mmu_enabled;
> +extern bool _mmu_enabled;
>
>  #define MAX_EARLY_MAPPING_SIZE SZ_128M
>
> @@ -236,7 +236,7 @@ void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
>
>  static pte_t *__init get_pte_virt(phys_addr_t pa)
>  {
> -       if (mmu_enabled) {
> +       if (_mmu_enabled) {
>                 clear_fixmap(FIX_PTE);
>                 return (pte_t *)set_fixmap_offset(FIX_PTE, pa);
>         } else {
> @@ -250,7 +250,7 @@ static phys_addr_t __init alloc_pte(uintptr_t va)
>          * We only create PMD or PGD early mappings so we
>          * should never reach here with MMU disabled.
>          */
> -       BUG_ON(!mmu_enabled);
> +       BUG_ON(!_mmu_enabled);
>
>         return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
>  }
> @@ -281,7 +281,7 @@ pmd_t early_pmd[PTRS_PER_PMD * NUM_EARLY_PMDS] __initdata __aligned(PAGE_SIZE);
>
>  static pmd_t *__init get_pmd_virt(phys_addr_t pa)
>  {
> -       if (mmu_enabled) {
> +       if (_mmu_enabled) {
>                 clear_fixmap(FIX_PMD);
>                 return (pmd_t *)set_fixmap_offset(FIX_PMD, pa);
>         } else {
> @@ -293,7 +293,7 @@ static phys_addr_t __init alloc_pmd(uintptr_t va)
>  {
>         uintptr_t pmd_num;
>
> -       if (mmu_enabled)
> +       if (_mmu_enabled)
>                 return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
>
>         pmd_num = (va - PAGE_OFFSET) >> PGDIR_SHIFT;
> @@ -467,9 +467,6 @@ static void __init setup_vm_final(void)
>         phys_addr_t pa, start, end;
>         struct memblock_region *reg;
>
> -       /* Set mmu_enabled flag */
> -       mmu_enabled = true;
> -
>         /* Setup swapper PGD for fixmap */
>         create_pgd_mapping(swapper_pg_dir, FIXADDR_START,
>                            __pa_symbol(fixmap_pgd_next),
> --
> 2.20.1
>
>
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-- 
Regards,
Atish