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Documentation: Adds hypervisor CPUID information

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Currently we only implement function 4000_0000h. This will expand in the
future but this is all we have right now.
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Ryan Houdek 2022-03-21 22:46:48 -07:00
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# FEXCore custom CPUID functions
## 4000_0000h - Hypervisor information function
* Follows VMWare and Microsoft's hypervisor information proposal
* https://lwn.net/Articles/301888/
* https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/feature-discovery
* EAX - The maximum input value for the hypervisor CPUID information
** 4000_0000h - Only first leaf
* EBX - Hypervisor vendor ID signature
** 'FEXI' - 4958_4546h
* ECX - Hypervisor vendor ID signature
** 'FEXI' - 4958_4546h
* EDX - Hypervisor vendor ID signature
** 'EMU\0' - 0055_4d45h
* memcpy ebx:ecx:edx in to a 12 byte string to get 'FEXIFEXIEMU\0' for determining running under FEX
## 4000_0001h - 4000_000Fh
* **Reserved range**
* Returns zero until implemented
## 4000_0010h - 4FFF_FFFFh
* **Undefined**
* FEX-Emu will return zero until implemented