[PATCH v3 2/4] arm64: mte: Add asynchronous mode support

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

MTE provides an asynchronous mode for detecting tag exceptions. In
particular instead of triggering a fault the arm64 core updates a
register which is checked by the kernel after the asynchronous tag
check fault has occurred.

Add support for MTE asynchronous mode.

The exception handling mechanism will be added with a future patch.

Note: KASAN HW activates async mode via kasan.mode kernel parameter.
The default mode is set to synchronous.
The code that verifies the status of TFSR_EL1 will be added with a
future patch.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
---
 arch/arm64/kernel/mte.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
index 53a6d734e29b..df7a1ae26d7c 100644
--- a/arch/arm64/kernel/mte.c
+++ b/arch/arm64/kernel/mte.c
@@ -153,8 +153,30 @@ void mte_init_tags(u64 max_tag)
 
 void mte_enable_kernel(enum kasan_hw_tags_mode mode)
 {
-	/* Enable MTE Sync Mode for EL1. */
-	sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
+	const char *m;
+
+	/* Preset parameter values based on the mode. */
+	switch (mode) {
+	case KASAN_HW_TAGS_ASYNC:
+		/* Enable MTE Async Mode for EL1. */
+		sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_ASYNC);
+		m = "asynchronous";
+		break;
+	case KASAN_HW_TAGS_SYNC:
+		sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
+		m = "synchronous";
+		break;
+	default:
+		/*
+		 * kasan mode should be always set hence we should
+		 * not reach this condition.
+		 */
+		WARN_ON_ONCE(1);
+		return;
+	}
+
+	pr_info_once("MTE: enabled in %s mode at EL1\n", m);
+
 	isb();
 }
 
-- 
2.30.0