Re: [Xen-users] cpuid masking on Xen 4.1

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/03/16 11:46, Roger Pau Monné wrote:
> On Fri, 11 Mar 2016, Charles Koprowski wrote:
>> Hi,
>>
>> I'm trying to use cpuid masking on dom0 by passing cpuid_mask_ecx,
>> cpuid_mask_edx, cpuid_mask_ext_ecx and cpuid_mask_ext_edx options to
>> Xen command line.
>>
>> Unfortunately, it seems that masks are not applied by Xen at boot
>> time. The output of the cpuid command and "cat /proc/cpuinfo" remains
>> the same as if I wouldn't pass any cpuid_mask_* options.
>>
>> So I tried to debug the problem by recompiling Xen. Adding some printk
>> debug log to xen/arch/x86/cpu/intel.c showed  that set_cpuidmask()
>> isn't called during CPU init because cpu_has_cpuid_faulting equals 1
>> in my case.
>>
>> Forcing set_cpuidmask() to be executed just prints out "(XEN) Cannot
>> set CPU feature mask on CPU#" messages...
> Do you know where's this message coming from? set_cpuidmask doesn't have 
> any message with this text in current versions.
>
> Also, could you try with a newer version of Xen? Xen 4.1 has been 
> unsupported for a long time now.
>
>> So here are my questions :
>>
>> Do all CPUs support CPUID masking ? In my case I'm trying on Intel
>> Xeon E7-4850 v3

This is a Hawell processor, and doesn't have cpuid masking.  It has
cpuid faulting instead.

> I'm not really familiar with with cpuid leveling, but I think there's 
> indeed something wrong. I would expect that having cpu_has_cpuid_faulting 
> means we would do the masking somewhere else (in the fault handler), but I 
> cannot find any usages of opt_cpuid_mask_* anywhere outside of the intel 
> and amd cpu initialization files.

If you look in the XenServer patch queue
(https://github.com/xenserver/xen-4.6.pg/) at a changeset older than
b39aebaf663395714ffcff19e9707500a1580d3f, you will find
xen-apply-cpuid-mask-to-cpuid-faulting.patch which causes the cpuid
masks to apply to faulting as well.

All of this however has been superseded by my cpuid improvement series
which is currently at v3 and expected to land in Xen 4.7.

What are you actually trying to do?

~Andrew

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