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 <redacted>
---
 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.
+        */
+       ;
 }