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+.. SPDX-License-Identifier: GPL-2.0
+
+===============
+ACRN CPUID bits
+===============
+
+A guest VM running on an ACRN hypervisor can check some of its features using
+CPUID.
+
+ACRN cpuid functions are:
+
+function: 0x40000000
+
+returns::
+
+ eax = 0x40000010
+ ebx = 0x4e524341
+ ecx = 0x4e524341
+ edx = 0x4e524341
+
+Note that this value in ebx, ecx and edx corresponds to the string
+"ACRNACRNACRN". The value in eax corresponds to the maximum cpuid function
+present in this leaf, and will be updated if more functions are added in the
+future.
+
+function: define ACRN_CPUID_FEATURES (0x40000001)
+
+returns::
+
+ ebx, ecx, edx
+ eax = an OR'ed group of (1 << flag)
+
+where ``flag`` is defined as below:
+
+================================= =========== ================================
+flag value meaning
+================================= =========== ================================
+ACRN_FEATURE_PRIVILEGED_VM 0 guest VM is a privileged VM
+================================= =========== ================================
+
+function: 0x40000010
+
+returns::
+
+ ebx, ecx, edx
+ eax = (Virtual) TSC frequency in kHz.