RISC-V: Avoid using invalid intermediate translations

This is almost entirely a comment.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

arch/riscv/kernel/head.S

@@ -96,7 +96,9 @@ relocate:
 
 	/*
 	 * Load trampoline page directory, which will cause us to trap to
-	 * stvec if VA != PA, or simply fall through if VA == PA
+	 * stvec if VA != PA, or simply fall through if VA == PA.  We need a
+	 * full fence here because setup_vm() just wrote these PTEs and we need
+	 * to ensure the new translations are in use.
 	 */
 	la a0, trampoline_pg_dir
 	srl a0, a0, PAGE_SHIFT
@@ -115,8 +117,14 @@ relocate:
 	la gp, __global_pointer$
 .option pop
 
-	/* Switch to kernel page tables */
+	/*
+	 * Switch to kernel page tables.  A full fence is necessary in order to
+	 * avoid using the trampoline translations, which are only correct for
+	 * the first superpage.  Fetching the fence is guarnteed to work
+	 * because that first superpage is translated the same way.
+	 */
 	csrw CSR_SATP, a2
+	sfence.vma
 
 	ret