[PATCH 1/3] x86/cpu: Process all CPUID dependencies after identifying CPU info

Sean Christopherson
2022-12-03T00:40:36Z

Process all CPUID dependencies to ensure that a dependent is disabled if
one or more of its parent features is unsupported. As is, cpuid_deps is
processed if an only if a feature is explicitly disabled via
clear_cpu_cap(), which makes it annoying/dangerous to use cpuid_deps for
features whose parent(s) do not always have explicit processing.

E.g. VMX and SGX depend on the synthetic X86_FEATURE_MSR_IA32_FEAT_CTL,
but there is no common location to clear MSR_IA32_FEAT_CTL, and so
consumers of VMX and SGX are forced to check MSR_IA32_FEAT_CTL on top
of the dependent feature.

Manually clearing X86_FEATURE_MSR_IA32_FEAT_CTL is the obvious
alternative, but it's subtly more difficult that updating
init_ia32_feat_ctl(). CONFIG_IA32_FEAT_CTL depends on any of
CONFIG_CPU_SUP_{INTEL,CENATUR,ZHAOXIN}, but init_ia32_feat_ctl() is
invoked if and only if the actual CPU type matches one of the
aforementioned CPU_SUP_* types. E.g. running a kernel built with

CONFIG_CPU_SUP_INTEL=y
CONFIG_CPU_SUP_AMD=y
# CONFIG_CPU_SUP_HYGON is not set
# CONFIG_CPU_SUP_CENTAUR is not set
# CONFIG_CPU_SUP_ZHAOXIN is not set

on a Cenatur or Zhaoxin CPU will leave X86_FEATURE_VMX set but not set
X86_FEATURE_MSR_IA32_FEAT_CTL, and will never call init_ia32_feat_ctl()
to give the kernel a convenient opportunity to clear
X86_FEATURE_MSR_IA32_FEAT_CTL.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/cpufeature.h | 1 +
arch/x86/kernel/cpu/common.c | 6 ++++++
arch/x86/kernel/cpu/cpuid-deps.c | 10 ++++++++++
3 files changed, 17 insertions(+)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 1a85e1fb0922..c4408d03b180 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -147,6 +147,7 @@ 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 apply_cpuid_deps(struct cpuinfo_x86 *c);

#define setup_force_cpu_cap(bit) do { \
set_cpu_cap(&boot_cpu_data, bit); \
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index bf4ac1cb93d7..094fc69dba63 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1887,6 +1887,12 @@ static void identify_cpu(struct cpuinfo_x86 *c)

ppin_init(c);

+ /*
+ * Apply CPUID dependencies to ensure dependent features are disabled
+ * if a parent feature is unsupported but wasn't explicitly disabled.
+ */
+ apply_cpuid_deps(c);
+
/* Init Machine Check Exception if available. */
mcheck_cpu_init(c);

diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index c881bcafba7d..68e26d4c8063 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -138,3 +138,13 @@ void setup_clear_cpu_cap(unsigned int feature)
{
do_clear_cpu_cap(NULL, feature);
}
+
+void apply_cpuid_deps(struct cpuinfo_x86 *c)
+{
+ const struct cpuid_dep *d;
+
+ for (d = cpuid_deps; d->feature; d++) {
+ if (!cpu_has(c, d->depends))
+ clear_cpu_cap(c, d->feature);
+ }
+}
--
2.39.0.rc0.267.gcb52ba06e7-goog