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

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Message-Id: <20201201090023.2852-1-vitaly.wool@konsulko.com>
Date:   Tue,  1 Dec 2020 11:00:24 +0200
From:   vitaly.wool@...sulko.com
To:     linux-riscv@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, Bin Meng <bin.meng@...driver.com>,
        Anup Patel <anup@...infault.org>,
        Alistair Francis <alistair.francis@....com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Vitaly Wool <vitaly.wool@...sulko.com>
Subject: [PATCH] riscv: toggle mmu_enabled flag in a precise manner

From: Vitaly Wool <vitaly.wool@...sulko.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.

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@...sulko.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