arch/x86/cpuinfo.rst

4 x86 Feature Flags
10 The list of feature flags in /proc/cpuinfo is not complete and
11 represents an ill-fated attempt from long time ago to put feature flags
14 However, the amount of feature flags is growing by the CPU generation,
17 What is more, those feature flags do not even need to be in that file
21 And even if it doesn't show a particular feature flag - although the CPU
24 feature and figure out if it is supported or not, regardless of whether
32 kernel has *enabled* and *supports*. As in: the CPUID feature flag is
38 So, if users want to know if a feature is available on a given system,
42 * the kernel knows about the feature enough to have an X86_FEATURE bit
47 * if the flag represents a hardware feature the hardware supports it.
52 On the one hand, a feature like "vaes" might be fully available to user
70 If the kernel cares about a feature or KVM want to expose the feature to
76 feature flags.
79 How are feature flags created?
82 Feature flags can be derived from the contents of CPUID leaves
85 These feature definitions are organized mirroring the layout of CPUID
88 If a feature is defined with a X86_FEATURE_<name> definition in
98 if a given feature is present. This is done in init_scattered_cpuid_features().
105 has only one feature and would waste 31 bits of space in the x86_capability[]
116 the feature X86_FEATURE_SPLIT_LOCK_DETECT will be enabled and
123 software feature implemented in the kernel. For example, Kernel Page Table
124 Isolation is purely software feature and its feature flag X86_FEATURE_PTI is
139 Feature flags are omitted by default from /proc/cpuinfo as it does not make
140 sense for the feature to be exposed to userspace in most cases. For example,
142 kernel feature used in the alternative runtime patching functionality. So the
164 For example, when a new kernel is running on old hardware or the feature is
166 problem enabling the feature at run time, the flag will not be displayed.
178 Even though the feature will still be detected via CPUID, the kernel disables
181 The feature is disabled at boot-time
183 A feature can be disabled either using a command-line parameter or because
185 to disable features using the feature number as defined in
195 The feature was known to be non-functional
198 The feature was known to be non-functional because a dependency was
199 missing at runtime. For example, AVX flags will not show up if XSAVE feature
200 is disabled since they depend on XSAVE feature. Another example would be broken
202 enable a feature.