[PATCH RFC/RFT v2 4/4] riscv: Stop emitting preventive sfence.vma for new userspace mappings with Svvptc

[Alexandre Ghiti]

The preventive sfence.vma were emitted because new mappings must be made
visible to the page table walker but Svvptc guarantees that xRET act as
a fence, so no need to sfence.vma for the uarchs that implement this
extension.

This allows to drastically reduce the number of sfence.vma emitted:

* Ubuntu boot to login:
Before: ~630k sfence.vma
After:  ~200k sfence.vma

* ltp - mmapstress01
Before: ~45k
After:  ~6.3k

* lmbench - lat_pagefault
Before: ~665k
After:   832 (!)

* lmbench - lat_mmap
Before: ~546k
After:   718 (!)

Signed-off-by: Alexandre Ghiti <alexgh...@rivosinc.com>
---
 arch/riscv/include/asm/pgtable.h | 16 +++++++++++++++-
 arch/riscv/mm/pgtable.c          | 13 +++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
index 0c94260b5d0c..50986e4c4601 100644
--- a/arch/riscv/include/asm/pgtable.h
+++ b/arch/riscv/include/asm/pgtable.h
@@ -473,6 +473,9 @@ static inline void update_mmu_cache_range(struct vm_fault 
*vmf,
                struct vm_area_struct *vma, unsigned long address,
                pte_t *ptep, unsigned int nr)
 {
+       asm_volatile_goto(ALTERNATIVE("nop", "j %l[svvptc]", 0, 
RISCV_ISA_EXT_SVVPTC, 1)
+                         : : : : svvptc);
+
        /*
         * The kernel assumes that TLBs don't cache invalid entries, but
         * in RISC-V, SFENCE.VMA specifies an ordering constraint, not a
@@ -482,12 +485,23 @@ static inline void update_mmu_cache_range(struct vm_fault 
*vmf,
         */
        while (nr--)
                local_flush_tlb_page(address + nr * PAGE_SIZE);
+
+svvptc:
+       /*
+        * Svvptc guarantees that xRET act as a fence, so when the uarch does
+        * not cache invalid entries, we don't have to do anything.
+        */
+       ;
 }
 #define update_mmu_cache(vma, addr, ptep) \
        update_mmu_cache_range(NULL, vma, addr, ptep, 1)
 
 #define __HAVE_ARCH_UPDATE_MMU_TLB
-#define update_mmu_tlb update_mmu_cache
+static inline void update_mmu_tlb(struct vm_area_struct *vma,
+                                 unsigned long address, pte_t *ptep)
+{
+       flush_tlb_range(vma, address, address + PAGE_SIZE);
+}
 
 static inline void update_mmu_cache_pmd(struct vm_area_struct *vma,
                unsigned long address, pmd_t *pmdp)
diff --git a/arch/riscv/mm/pgtable.c b/arch/riscv/mm/pgtable.c
index ef887efcb679..99ed389e4c8a 100644
--- a/arch/riscv/mm/pgtable.c
+++ b/arch/riscv/mm/pgtable.c
@@ -9,6 +9,9 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
                          unsigned long address, pte_t *ptep,
                          pte_t entry, int dirty)
 {
+       asm_volatile_goto(ALTERNATIVE("nop", "j %l[svvptc]", 0, 
RISCV_ISA_EXT_SVVPTC, 1)
+                         : : : : svvptc);
+
        if (!pte_same(ptep_get(ptep), entry))
                __set_pte_at(ptep, entry);
        /*
@@ -16,6 +19,16 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
         * the case that the PTE changed and the spurious fault case.
         */
        return true;
+
+svvptc:
+       if (!pte_same(ptep_get(ptep), entry)) {
+               __set_pte_at(ptep, entry);
+               /* Here only not svadu is impacted */
+               flush_tlb_page(vma, address);
+               return true;
+       }
+
+       return false;
 }
 
 int ptep_test_and_clear_young(struct vm_area_struct *vma,
-- 
2.39.2