linux-kernel - [PATCH 2/5] ARCv2: entry: push out the Z flag unclobber from common EXCEPTION

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Message-ID: <1558121528-30184-3-git-send-email-vgupta@synopsys.com>
Date:   Fri, 17 May 2019 12:32:05 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     <linux-snps-arc@...ts.infradead.org>
CC:     <linux-kernel@...r.kernel.org>,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>
Subject: [PATCH 2/5] ARCv2: entry: push out the Z flag unclobber from common EXCEPTION_PROLOGUE

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@...opsys.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