[RFC PATCH] x86/cpufeature: Add feature dependency checks

[Sohil Mehta]

Currently, the cpuid-deps[] table is only exercised when a particular
feature gets explicitly disabled and clear_cpu_cap() is called. However,
some of these listed dependencies might already be missing during boot.
Unexpected failures can occur when the kernel tries to use such a
feature.

Therefore, add boot time checks for missing feature dependencies and
disable any feature whose dependencies are not met.

Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
---
Arguably, this situation should only happen on broken hardware and it may not
make sense to add such a check to the kernel. OTOH, this can be viewed as a
safety mechanism to make failures more graceful on such configurations in real
or virtual environments.

I feel since we already have the cpuid-deps[] table and the incremental changes
are small, this patch might be a useful addition.

Also, if this check seems worthwhile, would it be useful to combine and rewrite
it with filter_cpuid_features() since it tries to do something similar?
---

 arch/x86/include/asm/cpufeature.h |  1 +
 arch/x86/kernel/cpu/common.c      |  4 ++++
 arch/x86/kernel/cpu/cpuid-deps.c  | 10 ++++++++++
 3 files changed, 15 insertions(+)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 0b9611da6c53..347ef04f65ef 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -148,6 +148,7 @@ extern const char * const x86_bug_flags[NBUGINTS*32];
 
 extern void setup_clear_cpu_cap(unsigned int bit);
 extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
+extern void filter_feature_dependencies(struct cpuinfo_x86 *c);
 
 #define setup_force_cpu_cap(bit) do {			\
 							\
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index d4e539d4e158..6b725dbd8db7 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1602,6 +1602,7 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
 
 		c->cpu_index = 0;
 		filter_cpuid_features(c, false);
+		filter_feature_dependencies(c);
 
 		if (this_cpu->c_bsp_init)
 			this_cpu->c_bsp_init(c);
@@ -1854,6 +1855,9 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	/* Filter out anything that depends on CPUID levels we don't have */
 	filter_cpuid_features(c, true);
 
+	/* Filter out features that don't have their dependencies met */
+	filter_feature_dependencies(c);
+
 	/* If the model name is still unset, do table lookup. */
 	if (!c->x86_model_id[0]) {
 		const char *p;
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index b7d9f530ae16..88b34a97278a 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -147,3 +147,13 @@ void setup_clear_cpu_cap(unsigned int feature)
 {
 	do_clear_cpu_cap(NULL, feature);
 }
+
+void filter_feature_dependencies(struct cpuinfo_x86 *c)
+{
+	const struct cpuid_dep *d;
+
+	for (d = cpuid_deps; d->feature; d++) {
+		if (boot_cpu_has(d->feature) && !boot_cpu_has(d->depends))
+			do_clear_cpu_cap(c, d->feature);
+	}
+}
-- 
2.34.1