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

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Hi Jisheng,

On 31/12/2023 07:21, Alexandre Ghiti wrote:
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();


I sent a patch for that here https://lore.kernel.org/linux-riscv/20240128120405.25876-1-alexghiti@xxxxxxxxxxxx/

You can add:

Reviewed-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>

Thanks,

Alex



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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