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Subject[PATCH v4 1/9] arm64: cpufeature: treat unknown fields as RES0
From: Mark Rutland <>

Any fields not defined in an arm64_ftr_bits entry are propagated to the
system-wide register value in init_cpu_ftr_reg(), and while we require
that these strictly match for the sanity checks, we don't update them in

Generally, the lack of an arm64_ftr_bits entry indicates that the bits
are currently RES0 (as is the case for the upper 32 bits of all
supposedly 32-bit registers).

A better default would be to use zero for the system-wide value of
unallocated bits, making all register checking consistent, and allowing
for subsequent simplifications to the arm64_ftr_bits arrays.

This patch updates init_cpu_ftr_reg() to treat unallocated bits as RES0
for the purpose of the system-wide safe value. These bits will still be
sanity checked with strict match requirements, as is currently the case.

Signed-off-by: Mark Rutland <>
Cc: Catalin Marinas <>
Cc: Will Deacon <>
Reviewed-by: Suzuki K Poulose <>
Reviewed-by: Catalin Marinas <>
Signed-off-by: Suzuki K Poulose <>
arch/arm64/kernel/cpufeature.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index fdf8f04..ea02201 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -410,23 +410,33 @@ static void __init sort_ftr_regs(void)
* Initialise the CPU feature register from Boot CPU values.
* Also initiliases the strict_mask for the register.
+ * Any bits that are not covered by an arm64_ftr_bits entry are considered
+ * RES0 for the system-wide value, and must strictly match.
static void __init init_cpu_ftr_reg(u32 sys_reg, u64 new)
u64 val = 0;
u64 strict_mask = ~0x0ULL;
+ u64 valid_mask = 0;
const struct arm64_ftr_bits *ftrp;
struct arm64_ftr_reg *reg = get_arm64_ftr_reg(sys_reg);


for (ftrp = reg->ftr_bits; ftrp->width; ftrp++) {
+ u64 ftr_mask = arm64_ftr_mask(ftrp);
s64 ftr_new = arm64_ftr_value(ftrp, new);

val = arm64_ftr_set_value(ftrp, val, ftr_new);
+ valid_mask |= ftr_mask;
if (!ftrp->strict)
- strict_mask &= ~arm64_ftr_mask(ftrp);
+ strict_mask &= ~ftr_mask;
+ val &= valid_mask;
reg->sys_val = val;
reg->strict_mask = strict_mask;
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