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

Nicholas Piggin [off-list ref] writes:

On Mon, 19 Mar 2018 14:43:00 +0530
"Naveen N. Rao" [off-list ref] wrote:

quoted

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