[PATCH v11 3/4] x86/cpu: Do a sanity check on required feature bits

[Maciej Wieczor-Retman]

From: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>

After CPU identification concludes, do a sanity check by comparing the
final x86_capability bitmask with the pre-defined required feature bits.

Because for_each_set_bit() expects 64-bit values and feature bitmasks
are 32-bit use DECLARE_BITMAP() to avoid unaligned memory accesses if
NCAPINTS is ever an odd number.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
---
Changelog v11:
- Use DECLARE_BITMAP() on missing[] and recast to a u32 pointer for
  iterating. Do it to avoid unaligned memory accesses when later using
  for_each_set_bit() if NCAPINTS is going to ever become an odd number.
  missing[] was previously an u32 array and for_each_set_bit() works on
  unsigned long chunks of memory.
- Add a paragraph about the above to the patch message.
- Remove Peter's acked-by due to more changes.

Changelog v10:
- Shorten the comment before the sanity check.
- cpu -> CPU in the warning.
- NCAPINTS << 5 -> NCAPINTS * 32

Changelog v9:
- REQUIRED_MASK_INITIALIZER -> REQUIRED_MASK_INIT
- Redo the comments.
- Fix reverse xmas order.
- Inside for_each_set_bit: (void *) -> (unsigned long *).
- 16 -> X86_CAP_BUF_SIZE.

Changelog v6:
- Add Peter's acked-by tag.
- Rename patch subject to imperative form.
- Add a char buffer to the x86_cap_name() call.

 arch/x86/kernel/cpu/common.c | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 0e318f3d56cb..92159a0963c8 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2005,6 +2005,38 @@ const char *x86_cap_name(unsigned int bit, char *buf)
 	return buf;
 }
 
+/*
+ * As a sanity check compare the final x86_capability bitmask with the initial
+ * predefined required feature bits.
+ */
+static void verify_required_features(const struct cpuinfo_x86 *c)
+{
+	u32 required_features[NCAPINTS] = REQUIRED_MASK_INIT;
+	DECLARE_BITMAP(missing, NCAPINTS * 32);
+	char cap_buf[X86_CAP_BUF_SIZE];
+	u32 *missing_u32;
+	unsigned int i;
+	u32 error = 0;
+
+	memset(missing, 0, sizeof(missing));
+	missing_u32 = (u32 *)missing;
+
+	for (i = 0; i < NCAPINTS; i++) {
+		missing_u32[i] = ~c->x86_capability[i] & required_features[i];
+		error |= missing_u32[i];
+	}
+
+	if (!error)
+		return;
+
+	/* At least one required feature is missing */
+	pr_warn("CPU %d: missing required feature(s):", c->cpu_index);
+	for_each_set_bit(i, missing, NCAPINTS * 32)
+		pr_cont(" %s", x86_cap_name(i, cap_buf));
+	pr_cont("\n");
+	add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+}
+
 /*
  * This does the hard work of actually picking apart the CPU stuff...
  */
@@ -2134,6 +2166,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	mcheck_cpu_init(c);
 
 	numa_add_cpu(smp_processor_id());
+
+	verify_required_features(c);
 }
 
 /*
-- 
2.53.0