Re: General protection fault in `switch_mm_irqs_off()`

Lendacky, Thomas
2019-01-09T14:29:47Z

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
>