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Date: Tue, 20 Mar 2018 10:05:59 +1000
From: Nicholas Piggin <npiggin@gmail.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>, Steven Rostedt
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 linuxppc-dev@lists.ozlabs.org, Benjamin Herrenschmidt
 <benh@kernel.crashing.org>, Anton Blanchard <anton@samba.org>,
 sathnaga@linux.vnet.ibm.com
Subject: Re: [PATCH 1/2] powerpc64/ftrace: Add a field in paca to disable
 ftrace in unsafe code paths
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On Tue, 20 Mar 2018 09:34:43 +1100
Michael Ellerman <mpe@ellerman.id.au> wrote:

> Nicholas Piggin <npiggin@gmail.com> writes:
> 
> > On Mon, 19 Mar 2018 14:43:00 +0530
> > "Naveen N. Rao" <naveen.n.rao@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.

Okay that's fine.

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

I'm not too concerned about some more u8 flags.

Thanks,
Nick