LoongArch: Improve hardware page table walker

LoongArch has similar problems explained in commit 7f0b1bf04511348995d6
("arm64: Fix barriers used for page table modifications"), when hardware
page table walker (PTW) enabled, speculative accesses may cause spurious
page fault in kernel space. Theoretically, in order to completely avoid
spurious page fault we need a "dbar + ibar" pair between the page table
modifications and the subsequent memory accesses using the corresponding
virtual address. But "ibar" is too heavy for performace, so we only use
a "dbar 0b11000" in set_pte(). And let spurious_fault() filter the rest
rare spurious page faults which should be avoided by "ibar".

Besides, we replace the llsc loop with amo in set_pte() which has better
performace, and refactor mmu_context.h to 1) avoid any load/store/branch
instructions between the writing of CSR.ASID & CSR.PGDL, 2) ensure flush
tlb operation is after updating ASID.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

1 files changed, 41 insertions, 0 deletions

diff --git a/arch/loongarch/mm/fault.c b/arch/loongarch/mm/fault.c
index 97b40defde06..deefd9617d00 100644
--- a/arch/loongarch/mm/fault.c
+++ b/arch/loongarch/mm/fault.c
@@ -31,11 +31,52 @@
 
 int show_unhandled_signals = 1;
 
+static int __kprobes spurious_fault(unsigned long write, unsigned long address)
+{
+	pgd_t *pgd;
+	p4d_t *p4d;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+
+	if (!(address & __UA_LIMIT))
+		return 0;
+
+	pgd = pgd_offset_k(address);
+	if (!pgd_present(pgdp_get(pgd)))
+		return 0;
+
+	p4d = p4d_offset(pgd, address);
+	if (!p4d_present(p4dp_get(p4d)))
+		return 0;
+
+	pud = pud_offset(p4d, address);
+	if (!pud_present(pudp_get(pud)))
+		return 0;
+
+	pmd = pmd_offset(pud, address);
+	if (!pmd_present(pmdp_get(pmd)))
+		return 0;
+
+	if (pmd_leaf(*pmd)) {
+		return write ? pmd_write(pmdp_get(pmd)) : 1;
+	} else {
+		pte = pte_offset_kernel(pmd, address);
+		if (!pte_present(ptep_get(pte)))
+			return 0;
+
+		return write ? pte_write(ptep_get(pte)) : 1;
+	}
+}
+
 static void __kprobes no_context(struct pt_regs *regs,
 			unsigned long write, unsigned long address)
 {
 	const int field = sizeof(unsigned long) * 2;
 
+	if (spurious_fault(write, address))
+		return;
+
 	/* Are we prepared to handle this kernel fault?	 */
 	if (fixup_exception(regs))
 		return;