arm64: cpufeature: Allow early filtering of feature override

Some CPUs are broken enough that some overrides need to be rejected
at the earliest opportunity. In some cases, that's right at cpu
feature override time.

Provide the necessary infrastructure to filter out overrides,
and to report such filtered out overrides to the core cpufeature code.

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210408131010.1109027-2-maz@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

2 files changed, 19 insertions, 0 deletions

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 066030717a4c..6de15deaa912 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -809,6 +809,12 @@ static void __init init_cpu_ftr_reg(u32 sys_reg, u64 new)
 					reg->name,
 					ftrp->shift + ftrp->width - 1,
 					ftrp->shift, str, tmp);
+		} else if ((ftr_mask & reg->override->val) == ftr_mask) {
+			reg->override->val &= ~ftr_mask;
+			pr_warn("%s[%d:%d]: impossible override, ignored\n",
+				reg->name,
+				ftrp->shift + ftrp->width - 1,
+				ftrp->shift);
 		}
 
 		val = arm64_ftr_set_value(ftrp, val, ftr_new);
diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index 83f1c4b92095..be92fcd319a1 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -25,6 +25,7 @@ struct ftr_set_desc {
 	struct {
 		char			name[FTR_DESC_FIELD_LEN];
 		u8			shift;
+		bool			(*filter)(u64 val);
 	} 				fields[];
 };
 
@@ -124,6 +125,18 @@ static void __init match_options(const char *cmdline)
 			if (find_field(cmdline, regs[i], f, &v))
 				continue;
 
+			/*
+			 * If an override gets filtered out, advertise
+			 * it by setting the value to 0xf, but
+			 * clearing the mask... Yes, this is fragile.
+			 */
+			if (regs[i]->fields[f].filter &&
+			    !regs[i]->fields[f].filter(v)) {
+				regs[i]->override->val  |= mask;
+				regs[i]->override->mask &= ~mask;
+				continue;
+			}
+
 			regs[i]->override->val  &= ~mask;
 			regs[i]->override->val  |= (v << shift) & mask;
 			regs[i]->override->mask |= mask;