[PATCH 1/2] powerpc64/ftrace: Add a field in paca to disable ftrace in unsafe code paths

[Naveen N. Rao]

Michael Ellerman wrote:
> Nicholas Piggin <npiggin at gmail.com> writes:
> 
>> On Mon, 19 Mar 2018 14:43:00 +0530
>> "Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> wrote:
>>
>>> We have some C code that we call into from real mode where we cannot
>>> take any exceptions. Though the C functions themselves are mostly safe,
>>> if these functions are traced, there is a possibility that we may take
>>> an exception. For instance, in certain conditions, the ftrace code uses
>>> WARN(), which uses a 'trap' to do its job.
>>> 
>>> For such scenarios, introduce a new field in paca 'ftrace_disabled',
>>> which is checked on ftrace entry before continuing. This field can then
>>> be set to a non-zero value to disable/pause ftrace, and reset to zero to
>>> resume ftrace.
>>> 
>>> Since KVM is the only user for this currently, we guard the
>>> ftrace/mcount checks within CONFIG_KVM. This can later be removed
>>> if/when there are other users.
>>
>> Why not test HSTATE_IN_GUEST then? Add ftrace_disabled if non-KVM users
>> come along.
> 
> We want to use it for the kexec down path, we've already had bugs there.

I had looked at kexec and we seem to be disabling ftrace in 
machine_kexec(). Has that been problematic?

I didn't actually realize that you wanted to use this in kexec path as 
well.

> 
> So please keep the separate flag and pull it out of #ifdef KVM.

Sure.

- Naveen