linux-kernel - Re: General protection fault in `switch_mm_irqs

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Message-ID: <ea49acb9-8165-80a3-e6fe-b29123ad5795@amd.com>
Date:   Wed, 9 Jan 2019 14:29:41 +0000
From:   "Lendacky, Thomas" <Thomas.Lendacky@....com>
To:     Paul Menzel <pmenzel@...gen.mpg.de>,
        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()`

On 1/9/19 7:35 AM, Paul Menzel wrote:
> Dear Thomas,
> 
> 
> 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)?

Thanks,
Tom

> 
> Kind regards,
> 
> Paul
>