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

[Mark Rutland]

On Tue, Jun 30, 2026 at 11:38:55AM +0200, Linus Walleij wrote:
> From: Linus Walleij <linus.walleij at 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 at arm.com>
> Signed-off-by: Linus Walleij <linus.walleij at linaro.org>
> Acked-by: Mark Rutland <mark.rutland at 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

Looks like you might have forgotten something here. ;)

This looks good to me, so I reckon you should drop it into Russell's
patch tracker:

  https://www.armlinux.org.uk/developer/patches/

Mark.

> 
> 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 at kernel.org>
>