[PATCH v3] ARM: entry: expand comment in __switch_to

[Linus Walleij <linusw@kernel.org>]

From: Linus Walleij <linus.walleij@linaro.org>

As per discussion between the developers in the mail thread
linked, expand the comment in __switch_to so that readers
of the code understand what is going on.

Link: https://lore.kernel.org/linux-arm-kernel/ZxDh9biUbf9W8gNN@J2N7QTR9R3/
Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
---
Changes in v3:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v2: https://lore.kernel.org/r/20251216-comments-in-switch-to-v2-1-190d8741db14@kernel.org

Changes in v2:
- Rebased on v6.19-rc1
- Link to v1: https://lore.kernel.org/r/20241028-comments-in-switch-to-v1-0-7280d09671a8@linaro.org
---
 arch/arm/kernel/entry-armv.S | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index a3d050ce9b79..2d2ea3ca880c 100644
--- a/arch/arm/kernel/entry-armv.S
+++ b/arch/arm/kernel/entry-armv.S
@@ -557,9 +557,16 @@ ENTRY(__switch_to)
 	ldmia	r4, {r4 - sl, fp, ip, lr}	@ Load all regs saved previously
 #ifdef CONFIG_VMAP_STACK
 	@
-	@ Do a dummy read from the new stack while running from the old one so
-	@ that we can rely on do_translation_fault() to fix up any stale PMD
-	@ entries covering the vmalloc region.
+	@ For a non-lazy mm switch, check_vmalloc_seq() has ensured that
+	@ that the active mm's page tables have mappings for the prev
+	@ task's stack and the next task's stack.
+	@
+	@ For a lazy mm switch the active mm's page tables have mappings
+	@ for the prev task's stack but might not have mappings for the
+	@ new task's stack. Do a dummy read from the new stack while
+	@ running from the old stack so that we can rely on
+	@ do_translation_fault() to populate missing PMD entries covering the
+	@ new task's stack in the old task's page tables.
 	@
 	ldr	r2, [ip]
 #ifdef CONFIG_KASAN_VMALLOC

---
base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
change-id: 20241028-comments-in-switch-to-0e24480e8495

Best regards,
--  
Linus Walleij <linusw@kernel.org>