Re: [PATCH v2 09/12] arm64: traps: Fix inconsistent faulting instruction skipping

[Mark Rutland <mark.rutland@arm.com>]

On Tue, Oct 15, 2019 at 04:21:09PM +0100, Dave Martin wrote:
> On Fri, Oct 11, 2019 at 04:24:53PM +0100, Mark Rutland wrote:
> > On Thu, Oct 10, 2019 at 07:44:37PM +0100, Dave Martin wrote:
> > > Correct skipping of an instruction on AArch32 works a bit
> > > differently from AArch64, mainly due to the different CPSR/PSTATE
> > > semantics.
> > > 
> > > There have been various attempts to get this right.  Currenty
> > > arm64_skip_faulting_instruction() mostly does the right thing, but
> > > does not advance the IT state machine for the AArch32 case.
> > > 
> > > arm64_compat_skip_faulting_instruction() handles the IT state
> > > machine but is local to traps.c, and porting other code to use it
> > > will make a mess since there are some call sites that apply for
> > > both the compat and native cases.
> > > 
> > > Since manual instruction skipping implies a trap, it's a relatively
> > > slow path.
> > > 
> > > So, make arm64_skip_faulting_instruction() handle both compat and
> > > native, and get rid of the arm64_compat_skip_faulting_instruction()
> > > special case.
> > > 
> > > Fixes: 32a3e635fb0e ("arm64: compat: Add CNTFRQ trap handler")
> > > Fixes: 1f1c014035a8 ("arm64: compat: Add condition code checks and IT advance")
> > > Fixes: 6436beeee572 ("arm64: Fix single stepping in kernel traps")
> > > Fixes: bd35a4adc413 ("arm64: Port SWP/SWPB emulation support from arm")
> > > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > > ---
> > >  arch/arm64/kernel/traps.c | 18 ++++++++----------
> > >  1 file changed, 8 insertions(+), 10 deletions(-)
> > 
> > This looks good to me; it's certainly easier to reason about.
> > 
> > I couldn't spot a place where we do the wrong thing today, given AFAICT
> > all the instances in arch/arm64/kernel/armv8_deprecated.c would be
> > UNPREDICTABLE within an IT block.
> > 
> > It might be worth calling out an example in the commit message to
> > justify the fixes tags.
> 
> IIRC I found no bug here; rather we have pointlessly fragmented code,
> so I followed the "if fixing the same bug in multiple places, merge
> those places so you need only fix it in one place next time" rule.

Sure thing, that makes sense to me.

> Since arm64_skip_faulting_instruction() is most of the way to being
> generically usable anyway, this series merges all the special-case
> handling into it.
> 
> I could add something like
> 
> --8<--
> 
> This allows this fiddly operation to be maintained in a single
> place rather than trying to maintain fragmented versions spread
> around arch/arm64.
> 
> -->8--
> 
> Any good?

My big concern is that the commit message reads as a fix, implying that
there's an existing correctness bug. I think that simplifying it to make
it clearer that it's a cleanup/improvement would be preferable.

How about:

| arm64: unify native/compat instruction skipping
|
| Skipping of an instruction on AArch32 works a bit differently from
| AArch64, mainly due to the different CPSR/PSTATE semantics.
|
| Currently arm64_skip_faulting_instruction() is only suitable for
| AArch64, and arm64_compat_skip_faulting_instruction() handles the IT
| state machine but is local to traps.c.
| 
| Since manual instruction skipping implies a trap, it's a relatively
| slow path.
| 
| So, make arm64_skip_faulting_instruction() handle both compat and
| native, and get rid of the arm64_compat_skip_faulting_instruction()
| special case.
|
| Signed-off-by: Dave Martin <Dave.Martin@arm.com>

With that, feel free to add:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

We could even point out that the armv8_deprecated cases are
UNPREDICTABLE in an IT block, and correctly emulated either way.

Thanks,
Mark.