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

[Michael Ellerman]

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.

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

If we're worried about space usage in the paca we can probably
consolidate this and some other things into a flags word.

cheers