Changing the CPUID hypervisor string for HVM DomU

Changing the CPUID hypervisor string for HVM DomU

[Fanny Dwargee]

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Hi,

I'm currently using Xen v4.6.1 compiled from sources on Linux Debian Jessie
and I would like to change the CPUID hypervisor vendor string when queried
from a HVM DomU (Windows7 SP1 64 bits).

According to http://www.sandpile.org/x86/cpuid.htm#level_4000_0000h this
information is queried via EAX=0x40000000 and returned on EBX-ECX-EDX
registers

The current vendor string is "XenVMMXenVMM" and I want to replace it by the
string "JUSTATEST" so I changed my DomU configuration to:

[...]
cpuid = [
'0x40000000:eax=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx,ebx=01010100010100110101010101001010,ecx=00000000000000000000000001010100,edx=01010011010001010101010001000001'
]

hap  = 1
acpi = 1
altp2mhvm = 1
[...]


but unfortunately the guest still shows "XenVMMXenVMM" as the hypervisor
string.

After looking for that string in the Xen sources it seems to be harcoded in
many places:

   - xen-4.6.1/docs/misc/pvrdtscp.c
   - xen-4.6.1/tools/firmware/seabios-dir-remote/src/fw/xen.c
   - xen-4.6.1/tools/firmware/hvmloader/hvmloader.c
   - xen-4.6.1/tools/misc/xen-detect.c
   - xen-4.6.1/unmodified_drivers/linux-2.6/platform-pci/platform-pci.c
   - xen-4.6.1/xen/include/public/arch-x86/cpuid.h
   - xen-4.6.1/extras/mini-os/include/xen/arch-x86/cpuid.h
   - xen-4.6.1/dist/install/usr/local/include/xen/arch-x86/cpuid.h

...so I'm afraid I will be unable to change it.

Any ideas or patches?

Thanks in advance

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Re: Changing the CPUID hypervisor string for HVM DomU

[Boris Ostrovsky]

On 03/28/2016 05:34 AM, Fanny Dwargee wrote:
> Hi,
>
> I'm currently using Xen v4.6.1 compiled from sources on Linux Debian 
> Jessie and I would like to change the CPUID hypervisor vendor string 
> when queried from a HVM DomU (Windows7 SP1 64 bits).
>
> According to http://www.sandpile.org/x86/cpuid.htm#level_4000_0000h 
> this information is queried via EAX=0x40000000 and returned on 
> EBX-ECX-EDX registers
>
> The current vendor string is "XenVMMXenVMM" and I want to replace it 
> by the string "JUSTATEST" so I changed my DomU configuration to:
>
>     [...]
>     cpuid = [
>     '0x40000000:eax=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx,ebx=01010100010100110101010101001010,ecx=00000000000000000000000001010100,edx=01010011010001010101010001000001'
>     ]
>
>     hap  = 1
>     acpi = 1
>     altp2mhvm = 1
>     [...]
>
>
> but unfortunately the guest still shows "XenVMMXenVMM" as the 
> hypervisor string.
>
> After looking for that string in the Xen sources it seems to be 
> harcoded in many places:
>
>   * xen-4.6.1/docs/misc/pvrdtscp.c
>   * xen-4.6.1/tools/firmware/seabios-dir-remote/src/fw/xen.c
>   * xen-4.6.1/tools/firmware/hvmloader/hvmloader.c
>   * xen-4.6.1/tools/misc/xen-detect.c
>   * xen-4.6.1/unmodified_drivers/linux-2.6/platform-pci/platform-pci.c
>   * xen-4.6.1/xen/include/public/arch-x86/cpuid.h
>   * xen-4.6.1/extras/mini-os/include/xen/arch-x86/cpuid.h
>   * xen-4.6.1/dist/install/usr/local/include/xen/arch-x86/cpuid.h
>
> ...so I'm afraid I will be unable to change it.
>
> Any ideas or patches?
>
> Thanks in advance


You can't change hypervisor signature (or most values in hypervisor 
leaves). See man page for xl.conf syntax ('cpuid' option) or 
libxc/xc_cpuid_x86.c:xc_cpuid_policy().

-boris

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