[PATCH v2] powerpc/code-patching: Speed up page mapping/unmapping on PPC32

[Christophe Leroy]

Since commit 591b4b268435 ("powerpc/code-patching: Pre-map patch area")
the patch area is premapped so intermediate page tables are already
allocated.

Use __set_pte_at() directly instead of the heavy map_kernel_page(),
at for unmapping just do a pte_clear() followed by a flush.

__set_pte_at() can be used directly without the filters in
set_pte_at() because we are mapping a normal page non executable.

Make sure gcc knows text_poke_area is page aligned in order to
optimise the flush.

This change reduces by 66% the time needed to activate ftrace on
an 8xx (588000 tb ticks instead of 1744000).

Don't perform the change on PPC64 for now, as it is problematic for
the time being.

Signed-off-by: Christophe Leroy <christophe.leroy at csgroup.eu>
---
v2: Only do it on PPC32 for now, mpe reported a problem on PPC64, see https://lore.kernel.org/lkml/165261053687.1047019.4165741740473209888.b4-ty@ellerman.id.au/T/#m9d91e820c43ebe56a72ad89403dac9eb270f5bb6
---
 arch/powerpc/lib/code-patching.c | 37 ++++++++++++++++++++++----------
 1 file changed, 26 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 6edf0697a526..01b9f5dc79d6 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -94,17 +94,20 @@ void __init poking_init(void)
 	static_branch_enable(&poking_init_done);
 }
 
+static unsigned long get_patch_pfn(void *addr)
+{
+	if (IS_ENABLED(CONFIG_MODULES) && is_vmalloc_or_module_addr(addr))
+		return vmalloc_to_pfn(addr);
+	else
+		return __pa_symbol(addr) >> PAGE_SHIFT;
+}
+
 /*
  * This can be called for kernel text or a module.
  */
 static int map_patch_area(void *addr, unsigned long text_poke_addr)
 {
-	unsigned long pfn;
-
-	if (IS_ENABLED(CONFIG_MODULES) && is_vmalloc_or_module_addr(addr))
-		pfn = vmalloc_to_pfn(addr);
-	else
-		pfn = __pa_symbol(addr) >> PAGE_SHIFT;
+	unsigned long pfn = get_patch_pfn(addr);
 
 	return map_kernel_page(text_poke_addr, (pfn << PAGE_SHIFT), PAGE_KERNEL);
 }
@@ -149,17 +152,29 @@ static int __do_patch_instruction(u32 *addr, ppc_inst_t instr)
 	int err;
 	u32 *patch_addr;
 	unsigned long text_poke_addr;
+	pte_t *pte;
+	unsigned long pfn = get_patch_pfn(addr);
 
-	text_poke_addr = (unsigned long)__this_cpu_read(text_poke_area)->addr;
+	text_poke_addr = (unsigned long)__this_cpu_read(text_poke_area)->addr & PAGE_MASK;
 	patch_addr = (u32 *)(text_poke_addr + offset_in_page(addr));
 
-	err = map_patch_area(addr, text_poke_addr);
-	if (err)
-		return err;
+	if (IS_ENABLED(CONFIG_PPC32)) {
+		pte = virt_to_kpte(text_poke_addr);
+		__set_pte_at(&init_mm, text_poke_addr, pte, pfn_pte(pfn, PAGE_KERNEL), 0);
+	} else {
+		err = map_patch_area(addr, text_poke_addr);
+		if (err)
+			return err;
+	}
 
 	err = __patch_instruction(addr, instr, patch_addr);
 
-	unmap_patch_area(text_poke_addr);
+	if (IS_ENABLED(CONFIG_PPC32)) {
+		pte_clear(&init_mm, text_poke_addr, pte);
+		flush_tlb_kernel_range(text_poke_addr, text_poke_addr + PAGE_SIZE);
+	} else {
+		unmap_patch_area(text_poke_addr);
+	}
 
 	return err;
 }
-- 
2.36.1