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Subject[PATCH 2/5] ARCv2: entry: push out the Z flag unclobber from common EXCEPTION_PROLOGUE
Date
Upon a taken interrupt/exception from User mode, HS hardware auto sets Z flag.
This helps shave a few instructions from EXCEPTION_PROLOGUE by eliding
re-reading ERSTATUS and some bit fiddling.

However TLB Miss Exception handler can clobber the CPU flags and still end
up in EXCEPTION_PROLOGUE in the slow path handling TLB handling case:

EV_TLBMissD
do_slow_path_pf
EV_TLBProtV (aliased to call_do_page_fault)
EXCEPTION_PROLOGUE

As a result, EXCEPTION_PROLOGUE need to "unclobber" the Z flag which this
patch changes. It is now pushed out to TLB Miss Exception handler.
The reasons beings:

- The flag restoration is only needed for slowpath TLB Miss Exception
handling, but currently being in EXCEPTION_PROLOGUE penalizes all
exceptions such as ProtV and syscall Trap, where Z flag is already
as expected.

- Pushing unclobber out to where it was clobbered is much cleaner and
also serves to document the fact.

- Makes EXCEPTION_PROLGUE similar to INTERRUPT_PROLOGUE so easier to
refactor the common parts which is what this series aims to do

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
---
arch/arc/include/asm/entry-arcv2.h | 8 --------
arch/arc/mm/tlbex.S | 11 +++++++++++
2 files changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/arc/include/asm/entry-arcv2.h b/arch/arc/include/asm/entry-arcv2.h
index 1c3520d1fa42..3209a6762960 100644
--- a/arch/arc/include/asm/entry-arcv2.h
+++ b/arch/arc/include/asm/entry-arcv2.h
@@ -225,14 +225,6 @@

; -- for interrupts, regs above are auto-saved by h/w in that order --
; Now do what ISR prologue does (manually save r12, sp, fp, gp, r25)
- ;
- ; Set Z flag if this was from U mode (expected by INTERRUPT_PROLOGUE)
- ; Although H/w exception micro-ops do set Z flag for U mode (just like
- ; for interrupts), it could get clobbered in case we soft land here from
- ; a TLB Miss exception handler (tlbex.S)
-
- and r10, r10, STATUS_U_MASK
- xor.f 0, r10, STATUS_U_MASK

INTERRUPT_PROLOGUE exception

diff --git a/arch/arc/mm/tlbex.S b/arch/arc/mm/tlbex.S
index 0e1e47a67c73..e50cac799a51 100644
--- a/arch/arc/mm/tlbex.S
+++ b/arch/arc/mm/tlbex.S
@@ -396,6 +396,17 @@ EV_TLBMissD_fast_ret: ; additional label for VDK OS-kit instrumentation
;-------- Common routine to call Linux Page Fault Handler -----------
do_slow_path_pf:

+#ifdef CONFIG_ISA_ARCV2
+ ; Set Z flag if exception in U mode. Hardware micro-ops do this on any
+ ; taken interrupt/exception, and thus is already the case at the entry
+ ; above, but ensuing code would have already clobbered.
+ ; EXCEPTION_PROLOGUE called in slow path, relies on correct Z flag set
+
+ lr r2, [erstatus]
+ and r2, r2, STATUS_U_MASK
+ bxor.f 0, r2, STATUS_U_BIT
+#endif
+
; Restore the 4-scratch regs saved by fast path miss handler
TLBMISS_RESTORE_REGS

--
2.7.4
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