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Subject: Re: [PATCH v5 13/19] x86/cpufeatures: Add flag to track whether MSR
 IA32_FEAT_CTL is configured
From:   Jacob Keller <jacob.e.keller@intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
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Organization: Intel Corporation
Message-ID: <df215c4c-82f0-5b15-57c3-d304fd94ff3b@intel.com>
Date:   Wed, 26 Feb 2020 12:41:09 -0800
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On 2/25/2020 4:58 PM, Jacob Keller wrote:
> On 2/25/2020 4:42 PM, Sean Christopherson wrote>> So there's something
> weird going on. Maybe "boot_cpu_has" in the
>>> vmx_disabled_by_bios is wrong? Hmm.
>>
>> Hmm, perhaps a bug somewhere else is overwriting the cpufeatures bit for
>> X86_FEATURE_VMX.  Let me see if I can reproduce from net-next.
>>
> 
> If you have any further suggestions for debugging, I'm happy to help try
> to figure this out. To my eyes, it looks like somehow bits get reset...
> It's definitely not clear to me how this happens.
> 
> There is the get_cpu_caps call.. but that seems to correctly call
> apply_forced_caps at the end.
> 
> That's all I have time for today.
> 
> Thanks,
> Jake
> 

Hi,

I kept digging into this, and I added a further print to the get_cpu_cap
function.

It looks like get_cpu_cap is being called again *after*
init_ia32_feat_ctl...

Digging further, I discovered this appears to be the call in setup_pku,
which would only be enabled for systems which have X86_FEATURE_PKU
enabled and supported. It's quite likely that test systems may not have
had this feature, hence why it went undetected till now.

Because of the extra get_cpu_cap call, the capabilities are reset. Since
we never use setup_clear_cpu_cap or pass NULL to clear_cpu_cap, the code
that sets the global cpu_caps_cleared bits is not run.

It's not clear to me what the best fix for this is.

Perhaps init_ia32_feat_ctl should be something run during
early_identify_cpu, since it's really checking global status (rdmsr),
and not per-CPU status. And then it could directly operate to call
setup_clear_cpu_cap, which would properly clear the bit globally,
ensuring that apply_forced_caps kicks in?

Or this needs to somehow be run *after* setup_pku? But that doesn't feel
very robust.

Thanks,
Jake