[PATCH v4 09/21] arm64: cpufeature: Add global feature override facility

[Marc Zyngier]

Add a facility to globally override a feature, no matter what
the HW says. Yes, this sounds dangerous, but we do respect the
"safe" value for a given feature. This doesn't mean the user
doesn't need to know what they are doing.

Nothing uses this yet, so we are pretty safe. For now.

Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
---
 arch/arm64/include/asm/cpufeature.h |  2 ++
 arch/arm64/kernel/cpufeature.c      | 44 +++++++++++++++++++++++++----
 2 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 9a555809b89c..465d2cb63bfc 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -75,6 +75,8 @@ struct arm64_ftr_reg {
 	u64				sys_val;
 	u64				user_val;
 	const struct arm64_ftr_bits	*ftr_bits;
+	u64				*override_val;
+	u64				*override_mask;
 };
 
 extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index e99eddec0a46..aaa075c6f029 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -544,13 +544,17 @@ static const struct arm64_ftr_bits ftr_raz[] = {
 	ARM64_FTR_END,
 };
 
-#define ARM64_FTR_REG(id, table) {		\
-	.sys_id = id,				\
-	.reg = 	&(struct arm64_ftr_reg){	\
-		.name = #id,			\
-		.ftr_bits = &((table)[0]),	\
+#define ARM64_FTR_REG_OVERRIDE(id, table, v, m) {		\
+		.sys_id = id,					\
+		.reg = 	&(struct arm64_ftr_reg){		\
+			.name = #id,				\
+			.ftr_bits = &((table)[0]),		\
+			.override_val = v,			\
+			.override_mask = m,			\
 	}}
 
+#define ARM64_FTR_REG(id, table) ARM64_FTR_REG_OVERRIDE(id, table, NULL, NULL)
+
 static const struct __ftr_reg_entry {
 	u32			sys_id;
 	struct arm64_ftr_reg 	*reg;
@@ -760,6 +764,7 @@ static void __init init_cpu_ftr_reg(u32 sys_reg, u64 new)
 	u64 strict_mask = ~0x0ULL;
 	u64 user_mask = 0;
 	u64 valid_mask = 0;
+	u64 override_val = 0, override_mask = 0;
 
 	const struct arm64_ftr_bits *ftrp;
 	struct arm64_ftr_reg *reg = get_arm64_ftr_reg(sys_reg);
@@ -767,9 +772,38 @@ static void __init init_cpu_ftr_reg(u32 sys_reg, u64 new)
 	if (!reg)
 		return;
 
+	if (reg->override_mask && reg->override_val) {
+		override_mask = *reg->override_mask;
+		override_val = *reg->override_val;
+	}
+
 	for (ftrp = reg->ftr_bits; ftrp->width; ftrp++) {
 		u64 ftr_mask = arm64_ftr_mask(ftrp);
 		s64 ftr_new = arm64_ftr_value(ftrp, new);
+		s64 ftr_ovr = arm64_ftr_value(ftrp, override_val);
+
+		if ((ftr_mask & override_mask) == ftr_mask) {
+			s64 tmp = arm64_ftr_safe_value(ftrp, ftr_ovr, ftr_new);
+			char *str = NULL;
+
+			if (ftr_ovr != tmp) {
+				/* Unsafe, remove the override */
+				*reg->override_mask &= ~ftr_mask;
+				*reg->override_val &= ~ftr_mask;
+				tmp = ftr_ovr;
+				str = "ignoring override";
+			} else if (ftr_new != tmp) {
+				/* Override was valid */
+				ftr_new = tmp;
+				str = "forced";
+			}
+
+			if (str)
+				pr_warn("%s[%d:%d]: %s to %llx\n",
+					reg->name,
+					ftrp->shift + ftrp->width - 1,
+					ftrp->shift, str, tmp);
+		}
 
 		val = arm64_ftr_set_value(ftrp, val, ftr_new);
 
-- 
2.29.2