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

Michael Ellerman wrote:

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

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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.
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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 t=

o

quoted

quoted

resume ftrace.
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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.

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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=20
machine_kexec(). Has that been problematic?

I didn't actually realize that you wanted to use this in kexec path as=20
well.

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

Sure.

- Naveen

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