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

[Nicholas Piggin]

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.

Thanks,
Nick