[RFC PATCH 03/25] KVM: arm64: Introduce a validation function for an ID register

[Reiji Watanabe]

Introduce arm64_check_features(), which does a basic validity checking
of an ID register value against the register's limit value that KVM
can support.
This function will be used by the following patches to check if an ID
register value that userspace tries to set can be supported by KVM on
the host.

Signed-off-by: Reiji Watanabe <reijiw at google.com>
---
 arch/arm64/include/asm/cpufeature.h |  1 +
 arch/arm64/kernel/cpufeature.c      | 26 ++++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index ef6be92b1921..eda7ddbed8cf 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -631,6 +631,7 @@ void check_local_cpu_capabilities(void);
 
 u64 read_sanitised_ftr_reg(u32 id);
 u64 __read_sysreg_by_encoding(u32 sys_id);
+int arm64_check_features(u32 sys_reg, u64 val, u64 limit);
 
 static inline bool cpu_supports_mixed_endian_el0(void)
 {
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 6ec7036ef7e1..d146ef759435 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -3114,3 +3114,29 @@ ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute *attr,
 		return sprintf(buf, "Vulnerable\n");
 	}
 }
+
+/*
+ * Check if all features that are indicated in the given ID register value
+ * ('val') are also indicated in the 'limit'.
+ */
+int arm64_check_features(u32 sys_reg, u64 val, u64 limit)
+{
+	struct arm64_ftr_reg *reg = get_arm64_ftr_reg(sys_reg);
+	const struct arm64_ftr_bits *ftrp;
+	u64 exposed_mask = 0;
+
+	if (!reg)
+		return -ENOENT;
+
+	for (ftrp = reg->ftr_bits; ftrp->width; ftrp++) {
+		if (arm64_ftr_value(ftrp, val) > arm64_ftr_value(ftrp, limit))
+			return -E2BIG;
+
+		exposed_mask |= arm64_ftr_mask(ftrp);
+	}
+
+	if (val & ~exposed_mask)
+		return -E2BIG;
+
+	return 0;
+}
-- 
2.33.0.882.g93a45727a2-goog