[PATCH 14/17] arm64/irq: Document handling of FEAT_NMI in irqflags.h

From: Mark Brown <[email protected]>

We have documentation at the top of irqflags.h which explains the DAIF
masking. Since the additional masking with NMIs is related and also
covers the IF in DAIF extend the comment to note what's going on with NMIs
though none of the code in irqflags.h is updated to handle NMIs.

Signed-off-by: Mark Brown <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jinjie Ruan <[email protected]>
---
 arch/arm64/include/asm/irqflags.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
index a8cb5a5c93b7..f2af9f423d88 100644
--- a/arch/arm64/include/asm/irqflags.h
+++ b/arch/arm64/include/asm/irqflags.h
@@ -18,6 +18,16 @@
  * always masked and unmasked together, and have no side effects for other
  * flags. Keeping to this order makes it easier for entry.S to know which
  * exceptions should be unmasked.
+ *
+ * With the addition of the FEAT_NMI extension we gain an additional
+ * class of superpriority IRQ/FIQ which is separately masked with a
+ * choice of modes controlled by SCTLR_ELn.{SPINTMASK,NMI}.  Linux
+ * sets SPINTMASK to 0 and NMI to 1 which results in ALLINT.ALLINT
+ * masking both superpriority interrupts and IRQ/FIQ regardless of the
+ * I and F settings. Since these superpriority interrupts are being
+ * used as NMIs we do not include them in the interrupt masking here,
+ * anything that requires that NMIs be masked needs to explicitly do
+ * so.
  */
 
 static __always_inline void __daif_local_irq_enable(void)
-- 
2.34.1