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

[Kevin Brodsky]

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.

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) | \

-- 
2.51.2