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ARCv2: entry: push out the Z flag unclobber from common EXCEPTION_PROLOGUE

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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>
This commit is contained in:
Vineet Gupta 2019-04-09 19:16:37 -07:00
parent 45869eb0c0
commit 23c0cbd0c7
2 changed files with 11 additions and 8 deletions
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8
arch/arc/include/asm/entry-arcv2.h
View File

@ -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

11
arch/arc/mm/tlbex.S
View File

@ -393,6 +393,17 @@ 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