In a very unsurprising turn of events, there is a large class of
firmware that is totally unable to deal with FEAT_NV3, and doesn't
set the required SCR2_EL3.NV3En bit, leading to an UNDEF exception
or an unhandled trap to EL3, depending on the implementation.

Allow the unfortunate user to override ID_AA64MMFR4_EL1.NV_frac
and get a working system. Hopefully firmware will be fixed before
actually HW ships, but I have been there before... :-/

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/cpufeature.h   |  1 +
 arch/arm64/kernel/cpufeature.c        |  4 +++-
 arch/arm64/kernel/image-vars.h        |  1 +
 arch/arm64/kernel/pi/idreg-override.c | 10 ++++++++++
 4 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index a57870fa96db5..a42683af79fb5 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -968,6 +968,7 @@ struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id);
 extern struct arm64_ftr_override id_aa64mmfr0_override;
 extern struct arm64_ftr_override id_aa64mmfr1_override;
 extern struct arm64_ftr_override id_aa64mmfr2_override;
+extern struct arm64_ftr_override id_aa64mmfr4_override;
 extern struct arm64_ftr_override id_aa64pfr0_override;
 extern struct arm64_ftr_override id_aa64pfr1_override;
 extern struct arm64_ftr_override id_aa64zfr0_override;
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 6ae1c816e2010..14fbfa8e6b7b5 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -785,6 +785,7 @@ static const struct arm64_ftr_bits ftr_raz[] = {
 struct arm64_ftr_override __read_mostly id_aa64mmfr0_override;
 struct arm64_ftr_override __read_mostly id_aa64mmfr1_override;
 struct arm64_ftr_override __read_mostly id_aa64mmfr2_override;
+struct arm64_ftr_override __read_mostly id_aa64mmfr4_override;
 struct arm64_ftr_override __read_mostly id_aa64pfr0_override;
 struct arm64_ftr_override __read_mostly id_aa64pfr1_override;
 struct arm64_ftr_override __read_mostly id_aa64zfr0_override;
@@ -858,7 +859,8 @@ static const struct __ftr_reg_entry {
 	ARM64_FTR_REG_OVERRIDE(SYS_ID_AA64MMFR2_EL1, ftr_id_aa64mmfr2,
 			       &id_aa64mmfr2_override),
 	ARM64_FTR_REG(SYS_ID_AA64MMFR3_EL1, ftr_id_aa64mmfr3),
-	ARM64_FTR_REG(SYS_ID_AA64MMFR4_EL1, ftr_id_aa64mmfr4),
+	ARM64_FTR_REG_OVERRIDE(SYS_ID_AA64MMFR4_EL1, ftr_id_aa64mmfr4,
+			       &id_aa64mmfr4_override),
 
 	/* Op1 = 0, CRn = 10, CRm = 4 */
 	ARM64_FTR_REG(SYS_MPAMIDR_EL1, ftr_mpamidr),
diff --git a/arch/arm64/kernel/image-vars.h b/arch/arm64/kernel/image-vars.h
index d4c7d45ae6bc8..d15c2cb1b0f28 100644
--- a/arch/arm64/kernel/image-vars.h
+++ b/arch/arm64/kernel/image-vars.h
@@ -51,6 +51,7 @@ PI_EXPORT_SYM(id_aa64isar2_override);
 PI_EXPORT_SYM(id_aa64mmfr0_override);
 PI_EXPORT_SYM(id_aa64mmfr1_override);
 PI_EXPORT_SYM(id_aa64mmfr2_override);
+PI_EXPORT_SYM(id_aa64mmfr4_override);
 PI_EXPORT_SYM(id_aa64pfr0_override);
 PI_EXPORT_SYM(id_aa64pfr1_override);
 PI_EXPORT_SYM(id_aa64smfr0_override);
diff --git a/arch/arm64/kernel/pi/idreg-override.c b/arch/arm64/kernel/pi/idreg-override.c
index bc57b290e5e7b..4e47616bcac23 100644
--- a/arch/arm64/kernel/pi/idreg-override.c
+++ b/arch/arm64/kernel/pi/idreg-override.c
@@ -106,6 +106,15 @@ static const struct ftr_set_desc mmfr2 __prel64_initconst = {
 	},
 };
 
+static const struct ftr_set_desc mmfr4 __prel64_initconst = {
+	.name		= "id_aa64mmfr4",
+	.override	= &id_aa64mmfr4_override,
+	.fields		= {
+		FIELD("nv_frac", ID_AA64MMFR4_EL1_NV_frac_SHIFT, NULL),
+		{}
+	},
+};
+
 static bool __init pfr0_sve_filter(u64 val)
 {
 	/*
@@ -220,6 +229,7 @@ PREL64(const struct ftr_set_desc, reg) regs[] __prel64_initconst = {
 	{ &mmfr0	},
 	{ &mmfr1	},
 	{ &mmfr2	},
+	{ &mmfr4	},
 	{ &pfr0 	},
 	{ &pfr1 	},
 	{ &isar1	},
-- 
2.47.3