Title   : How to mask CPUID feature flags by using Intel CPUID Feature Mask
Authors : Liping.ke@xxxxxxxxx
Reviewer:  
Created : June-22-2008
Updated : July-8-2008


Backgrounds for CPUID Feature Mask 
----------------------------------
Cpuid mask feature is intended to be used to limit the feature
flags reported by CPUID.1.EDX:ECX such that only those user 
designated feature flags are reported by this 
processor when CPUID.EAX=1 is executed. This feature will 
improve guest os applications compatibility.


How to turn on CPUID Feature Mask
----------------------------------
Please use cpuid_mask_ecx grub option to mask ecx reporting 
features and use cpuid_mask_edx grub option to mask edx 
reporting features.

For example, you can add below cpuid_mask_ecx or cpuid_mask_edx option 
in grub entry:

title Xen-Fedora Core (2.6.18-xen)
        root (hd0,0)
        kernel /boot/xen.gz com1=115200,8n1 console=com1 cpuid_mask_ecx = 0xFFFF7FFF cpuid_mask_edx = 0xFFFFFFFC
        module /boot/vmlinuz-2.6.18.8-xen root=LABEL=/ ro xencons=ttyS console=tty0 console=ttyS0
        module /boot/initrd-2.6.18-xen.img

In this setting, the 19th bit of ecx (SSE4.1) and 2nd bit of edx 
(Debugging Extensions) feature will be masked (corresponding bit is 0). 
When CPUID.EAX = 1 is executed, even if this cpu model supports 
these features, output ECX still reports the 19th bit is 0 and 
EDX reports the 2nd bit is 0, faking this cpu model does not suppport 
those features. If you don't care a feature, just let coresponding bit be 1.

For detailed cx/dx bit information, please refer to 
Intel Software Develop Manual 2A Figure 3.7 and Figure 3.8.


How to use (cpuid_mask_ecx, cpuid_mask_edx) value pair 
to rev_down a higher cpu model to a lower one
-------------------------------------------------------
For example, E8400[Intel? Core?2 Duo Processor series] ecx = 0x0008E3FD, 
edx = 0xBFEBFBFF when executing CPUID.EAX = 1 normally. If you want to rev_down
cpu model down to this E8400, you can just simply set cpuid_mask_ecx = 0x0008E3FD,
cpuid_mask_edx = 0xBFEBFBFF in grub entry.