[PATCH v2 1/2] arm64: Add override for MMFR1.HAFDBS

[Robin Murphy]

In general it might be nice to have the ability to disable hardware
access/dirty bit management for debugging or performance comparison
purposes without having to rebuild the kernel. However once FEAT_HAFT
comes into the picture we also start to have a real functional concern
where the decision to use HAFT based on the boot CPUs can prevent SVA
or late-onlining if SMMUs/CPUs are later found to lack HAFT support.

To that end, add the appropriate MMFR1 override, with an easy "nohaft"
alias for the significant case, partly since the feature/field naming
isn't the most obvious, but also so it could potentially be redirected
if someone wanted to attempt a higher-level means of turning off just
HAFT usage independently from FEAT_HDBSS in future.

Signed-off-by: Robin Murphy <robin.murphy at arm.com>
---

v2: New

OK, turns out this was actually incredibly easy, thanks Marc for
making the framework so idiot-proof!

 Documentation/admin-guide/kernel-parameters.txt | 3 +++
 arch/arm64/kernel/pi/idreg-override.c           | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index b5493a7f8f22..7f23e5b8dc44 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -565,6 +565,9 @@ Kernel parameters
 	arm64.nogcs	[ARM64] Unconditionally disable Guarded Control Stack
 			support
 
+	arm64.nohaft	[ARM64] Unconditionally disable Hardware managed Access
+			Flag for Table descriptors support
+
 	arm64.nomops	[ARM64] Unconditionally disable Memory Copy and Memory
 			Set instructions support
 
diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
index bc57b290e5e7..0e051fec5afe 100644
--- a/arch/arm64/kernel/pi/idreg-override.c
+++ b/arch/arm64/kernel/pi/idreg-override.c
@@ -64,6 +64,7 @@ static const struct ftr_set_desc mmfr1 __prel64_initconst = {
 	.override	= &id_aa64mmfr1_override,
 	.fields		= {
 		FIELD("vh", ID_AA64MMFR1_EL1_VH_SHIFT, mmfr1_vh_filter),
+		FIELD("hafdbs", ID_AA64MMFR1_EL1_HAFDBS_SHIFT, NULL),
 		{}
 	},
 };
@@ -246,6 +247,7 @@ static const struct {
 	{ "arm64.nomte",		"id_aa64pfr1.mte=0" },
 	{ "nokaslr",			"arm64_sw.nokaslr=1" },
 	{ "rodata=off",			"arm64_sw.rodataoff=1" },
+	{ "arm64.nohaft",		"id_aa64mmfr1.hafdbs=2" },
 	{ "arm64.nolva",		"id_aa64mmfr2.varange=0" },
 	{ "arm64.no32bit_el0",		"id_aa64pfr0.el0=1" },
 	{ "arm64.nompam",		"id_aa64pfr0.mpam=0 id_aa64pfr1.mpam_frac=0" },
-- 
2.54.0.dirty