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Date:   Wed, 9 Jan 2019 15:34:35 +0100
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Thomas Lendacky <Thomas.Lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Jiri Kosina <jikos@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: General protection fault in `switch_mm_irqs_off()`

Dear Thomas,


On 01/09/19 15:29, Lendacky, Thomas wrote:
> On 1/9/19 7:35 AM, Paul Menzel wrote:

>> On 01/09/19 14:16, Thomas Gleixner wrote:
>>
>>> On Wed, 9 Jan 2019, Paul Menzel wrote:
>>>> I get the same with microcode updates applied.
>>>>
>>>>     $ dmesg | grep 'microcode: CPU0: patch_level'
>>>>     [    3.809210] microcode: CPU0: patch_level=0x0600063e
>>>>     $ sudo modprobe msr
>>>>     $ sudo ./wrmsr 0x49 0x1
>>>>     wrmsr: CPU 0 cannot set MSR 0x00000049 to 0x0000000000000001
>>>>
>>>>> Could you please post /proc/cpuinfo from such a boot as well?
>>>
>>> /proc/cpuinfo unfortunately does not contain the amd specific IBPB flag,
>>> but the dmesg of the original report says:
>>>
>>>   Spectre V2 : mitigation: Enabling conditional Indirect Branch Prediction Barrier
>>>
>>> which means, that the CPUID bit is set.
>>>
>>> Can you please provide the output of:
>>>
>>>   cpuid -1 -l 0x80000008 -r
>>>
>>> On my AMD Opteron(TM) Processor 6276 with microcode 0x600063d loaded I get:
>>>
>>>    0x80000008 0x00: eax=0x00003030 ebx=0x00000000 ecx=0x0000500f edx=0x00000000
>>>
>>> EBX is 0, which means that X86_FEATURE_AMD_IBPB is not enabled. So the
>>> kernel does not try to use the speculation control MSR (0x49).
>>
>> With CPUID 20180519, I get the output below with both microcode update versions.
>>
>> 0x600063d:
>>
>>     $ sudo ./cpuid -1 -l 0x80000008 -r
>>     CPU:
>>        0x80000008 0x00: eax=0x00003030 ebx=0x00000000 ecx=0x0000500f edx=0x00000000
>>
>> 0x600063e:
>>
>>     $ sudo ./cpuid -1 -l 0x80000008 -r
>>     CPU:
>>        0x80000008 0x00: eax=0x00003030 ebx=0x00000000 ecx=0x0000500f edx=0x00000000
> 
> Hmmm... so ebx is 0 for both versions, so I'm not sure how IBPB is being
> set. What's the CPUID output for 0x07 (cpuid -1 -l 0x07 -r)?

    $ sudo ./cpuid -1 -l 0x07 -r
    CPU:
       0x00000007 0x00: eax=0x00000000 ebx=0x00000000 ecx=0x00000000 edx=0x00000000


Kind regards,

Paul


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