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Index > Windows > Why CPUID hypervisor flag is set?

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CandyMan



Joined: 04 Sep 2009
Posts: 345
Location: film "CandyMan" directed through Bernard Rose OR Candy Shop
CandyMan
Does anyone know why the processor returns
CPUID.00000001h.ECX = 0xFE9AE383 and the flag set
(Bit 31: Running on a hypervisor) on a real CPU?

(Windows OS, CPU = Intel Xeon ES-2680, family 6, model 62, stepping 4).

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Post 11 Aug 2021, 15:38
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bitRAKE



Joined: 21 Jul 2003
Posts: 3387
Location: vpcmipstrm
bitRAKE
AMD documentation states it as reserved, but read as zero. Obviously, the hypervisor controls what is returned for CPUID and can set it to any value.

This article suggests that the value is not well defined:
http://www.os2museum.com/wp/frequently-given-misleading-answers/

Maybe you could test from an EFI Shell to insure Windows is not doing something?

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Post 11 Aug 2021, 19:23
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sinsi



Joined: 10 Aug 2007
Posts: 742
Location: Adelaide
sinsi
If you have enabled Hyper-V in Windows, then even if you aren't using any VMs Windows runs as a special VM.
Quote:
with virtualization enabled, the host OS also runs on top of the Hyper-V virtualization layer, just as guest operating systems do.

https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/about/
Post 11 Aug 2021, 21:06
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Feryno



Joined: 23 Mar 2005
Posts: 479
Location: Czech republic, Slovak republic
Feryno
then you can then ask hypervisor info by executing cpuid with input eax=40000000h and further
more info here:
https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/tlfs
Post 20 Aug 2021, 05:11
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