[PATCH 1/2] arm64: mm: Add note about overlays in PIE_EL1

[Kevin Brodsky]

On 21/05/2026 11:42, Kevin Brodsky wrote:
> It isn't completely obvious why user page types do not have overlays
> applied in PIE_EL1. Add a comment to that effect, to avoid
> unpleasant surprises in the future.

Should probably s/PIE_EL1/PIR_EL1/ in this whole patch.

I'm also tempted to rename PIE_E* to PIR_E* to avoid anyone else's brain
being confused like mine... The extension is called PIE, but this is
really about the register configuration, i.e. PIR.

- Kevin

> Signed-off-by: Kevin Brodsky <kevin.brodsky at arm.com>
> ---
>  arch/arm64/include/asm/pgtable-prot.h | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
> index 212ce1b02e15..6e2f99820909 100644
> --- a/arch/arm64/include/asm/pgtable-prot.h
> +++ b/arch/arm64/include/asm/pgtable-prot.h
> @@ -175,6 +175,13 @@ static inline bool __pure lpa2_is_enabled(void)
>  	PIRx_ELx_PERM_PREP(pte_pi_index(_PAGE_READONLY),      PIE_R_O)   | \
>  	PIRx_ELx_PERM_PREP(pte_pi_index(_PAGE_SHARED),        PIE_RW_O))
>  
> +/*
> + * Regular user page types such as _PAGE_SHARED must not have overlays applied
> + * in PIE_EL1. If POE is enabled at EL1, and in the absence of FEAT_LSUI, this
> + * would break futex atomic operations on user memory with a non-default
> + * POIndex; the privileged atomic load/store instructions would be mistakenly
> + * checked against POR_EL1.
> + */
>  #define PIE_E1	( \
>  	PIRx_ELx_PERM_PREP(pte_pi_index(_PAGE_GCS),           PIE_NONE_O) | \
>  	PIRx_ELx_PERM_PREP(pte_pi_index(_PAGE_GCS_RO),        PIE_NONE_O) | \
>