Re: 4.4-rc5 Setting trap flag inside nmi handler results in HARD LOCKUP

[Jeff Merkey]

On 12/16/15, Jeff Merkey <linux.mdb@xxxxxxxxx> wrote:
> On 12/16/15, Jeff Merkey <linux.mdb@xxxxxxxxx> wrote:
>> Setting the (trap flag | resume flag) inside of an nmi handler results
>> in a hard lockup while setting the resume flag works fine.
>>
>> The watchdog detector fails to detect the lockup.  I am currently
>> examining the trap gate and interrupt gate setup on Linux and if
>> anyone has any ideas it would be nice to be able to debug and step
>> through the nmi handlers.  I got breakpoints to work.  I noticed
>> kgdb/kdb just punts here and refuses to allow someone to step inside
>> an nmi handler.
>>
>> There is no reason Linux should not allow this to work since windows
>> does and every other OS out there.  I have seen this across some rex64
>> sysret calls as well this lockup behavior.
>>
>> Anyone who is an intel expert with any clues would love some input if
>> you know about this problem.
>>
>> Jeff
>>
>
> More info.  Linux is getting a trap and it looks like the IDT is
> getting swapped when it gets it -- POW - Dead Linux.
>
> Damn ...  Well, it will be long night and lots of builds ...
>
> Jeff
>

>He is stepping into native_safe_halt() when this bug occurs (processor
>has halted).   I am starting to wonder is this is a linux bug or intel
>bug.  I am starting to lean towards intel bug possibly.  I will go and
>review intels documentation about what happens when a processor has
>been halted, is triggered with an NMI, then someone reloads the
>processor with the trap flag set then returns to a hlt instruction.

>Wow, this fucking cool .. I can debug Linus'  "I halt when idle" core
>function in the linux scheduler.

There is still a bug in there and its not that instruction.  I got the
halt instruction to work.  It has something to do with how linux
handles trap and interrupt gates if someone sets a trap flag.    Seems
pretty random as to where I see it.  Still working.

I will submit a patch when I find it.
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