[PATCH v2 5/7] ARM: switch_to: clean up Thumb2 code path

[Nicolas Pitre]

On Mon, 18 Oct 2021, Ard Biesheuvel wrote:

> The load-multiple instruction that essentially performs the switch_to
> operation in ARM mode, by loading all callee save registers as well the
> stack pointer and the program counter, is split into 3 separate loads
> for Thumb-2, with the IP register used as a temporary to capture the
> value of R4 before it gets overwritten.
> 
> We can clean this up a bit, by sticking with a single LDMIA instruction,
> but one that pops SP and PC into IP and LR, respectively, and by using
> ordinary move register and branch instructions to get those values into
> SP and PC. This also allows us to move the set_current call closer to
> the assignment of SP, reducing the window where those are mutually out
> of sync. This is especially relevant for CONFIG_VMAP_STACK, which is
> being introduced in a subsequent patch, where we need to issue a load
> that might fault from the new stack while running from the old one, to
> ensure that stale PMD entries in the VMALLOC space are synced up.

What could happen if they are of sync? The window is small but still.


Nicolas