riscv: mm: synchronize MMU after pte change

Because RISC-V compliant implementations can cache invalid entries
in TLB, an SFENCE.VMA is necessary after changes to the page table.
This patch adds an SFENCE.vma for the vmalloc_fault path.

Signed-off-by: ShihPo Hung <shihpo.hung@sifive.com>
[paul.walmsley@sifive.com: reversed tab->whitespace conversion,
 wrapped comment lines]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: linux-riscv@lists.infradead.org
Cc: stable@vger.kernel.org

arch/riscv/mm/fault.c

@@ -29,6 +29,7 @@
 
 #include <asm/pgalloc.h>
 #include <asm/ptrace.h>
+#include <asm/tlbflush.h>
 
 /*
  * This routine handles page faults.  It determines the address and the
@@ -278,6 +279,18 @@ vmalloc_fault:
 		pte_k = pte_offset_kernel(pmd_k, addr);
 		if (!pte_present(*pte_k))
 			goto no_context;
+
+		/*
+		 * The kernel assumes that TLBs don't cache invalid
+		 * entries, but in RISC-V, SFENCE.VMA specifies an
+		 * ordering constraint, not a cache flush; it is
+		 * necessary even after writing invalid entries.
+		 * Relying on flush_tlb_fix_spurious_fault would
+		 * suffice, but the extra traps reduce
+		 * performance. So, eagerly SFENCE.VMA.
+		 */
+		local_flush_tlb_page(addr);
+
 		return;
 	}
 }