Forcing old Microcode to the guest

[jen2]

VMware will load the host Microcode, which is the newest in my case, and I couldn't find a way in the VMware bios to load old Microcode file,

I want to test different Microcode in the guest, without downgrading my host BIOS  even though I can.

Is there any way to downgrade only the VMware guest with older Microcode.

[ender_]

The guest will always use the same microcode as the host, since it's not a resource you can configure per-guest (the guest runs on your host CPU, just partitioned from the host OS, and as such can't have a different microcode).

[jen2]

The guest will only use the host microcode only if the host has the newest microcode like in my case,

In case the host has an older Microcode, you can update the microcode file in the guest OS (Lib\firmware\intel-ucode in Linux, in Windows it the file the file mcupdate_GenuineIntel.dll ) and newer microcode can be loaded in the guest, and different from the host

Once VMware released a new Microcode https://flings.vmware.com/vmware-cpu-microcode-update-driver can be loaded isolated from the host Microcode if I am right

But I want to use older Microcode just to do some tests,

[ender_]

You normally can't update microcode from within a guest (the hypervisor will prevent it; VMWare's microcode update driver is intended to be used on the host). If you do somehow manage to update the microcode inside the guest, it'll affect the host as well, since the CPU doesn't have any way to run separate microcode for host and guest.

[jen2]

hmmmm, intriguing, when I have the time I will test this theory, I will downgrade my bios Microcode and try a newer Microcode in the guest and I will return,

The thing is the OS, regardless what OS it is, will always choose the newest Microcode upon booting, and in my case it is my laptop Microcode, in which I have modded manually to the latest Microcode

[jen2]

I guess you were right, I tried to use newer Microcode in the guest, both Linux and Windows and it didn't work, the guest used the loaded Microcode.

I will test again sometime to be more sure as I did it an hurry, but probably I will get the same results.