Re: [PATCH 1/4] riscv: tlb: fix __p*d_free_tlb()

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Hi Jisheng,

On 19/12/2023 18:50, Jisheng Zhang wrote:
If non-leaf PTEs I.E pmd, pud or p4d is modified, a sfence.vma is
a must for safe, imagine if an implementation caches the non-leaf
translation in TLB, although I didn't meet this HW so far, but it's
possible in theory.

Signed-off-by: Jisheng Zhang<jszhang@xxxxxxxxxx>
---
  arch/riscv/include/asm/pgalloc.h | 20 +++++++++++++++++---
  1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/include/asm/pgalloc.h b/arch/riscv/include/asm/pgalloc.h
index d169a4f41a2e..a12fb83fa1f5 100644
--- a/arch/riscv/include/asm/pgalloc.h
+++ b/arch/riscv/include/asm/pgalloc.h
@@ -95,7 +95,13 @@ static inline void pud_free(struct mm_struct *mm, pud_t *pud)
  		__pud_free(mm, pud);
  }
-#define __pud_free_tlb(tlb, pud, addr) pud_free((tlb)->mm, pud)
+#define __pud_free_tlb(tlb, pud, addr)					\
+do {									\
+	if (pgtable_l4_enabled) {					\
+		pagetable_pud_dtor(virt_to_ptdesc(pud));		\
+		tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(pud));	\


The specification indeed states that an sfence.vma must be emitted after a page directory modification. Your change is not enough though since eventually tlb_flush() is called and in this function we should add:

if (tlb->freed_tables)
    tlb_flush_mm();

otherwise we are not guaranteed that a "global" sfence.vma is called.

Would you be able to benchmark this change and see the performance impact?

Thanks,

Alex


+	}								\
+} while (0)
#define p4d_alloc_one p4d_alloc_one
  static inline p4d_t *p4d_alloc_one(struct mm_struct *mm, unsigned long addr)
@@ -124,7 +130,11 @@ static inline void p4d_free(struct mm_struct *mm, p4d_t *p4d)
  		__p4d_free(mm, p4d);
  }
-#define __p4d_free_tlb(tlb, p4d, addr) p4d_free((tlb)->mm, p4d)
+#define __p4d_free_tlb(tlb, p4d, addr)					\
+do {									\
+	if (pgtable_l5_enabled)						\
+		tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(p4d));	\
+} while (0)
  #endif /* __PAGETABLE_PMD_FOLDED */
static inline void sync_kernel_mappings(pgd_t *pgd)
@@ -149,7 +159,11 @@ static inline pgd_t *pgd_alloc(struct mm_struct *mm)
#ifndef __PAGETABLE_PMD_FOLDED -#define __pmd_free_tlb(tlb, pmd, addr) pmd_free((tlb)->mm, pmd)
+#define __pmd_free_tlb(tlb, pmd, addr)				\
+do {								\
+	pagetable_pmd_dtor(virt_to_ptdesc(pmd));		\
+	tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(pmd));	\
+} while (0)
#endif /* __PAGETABLE_PMD_FOLDED */




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