Commit 648d4e39 authored Apr 09, 2019 by Vineet Gupta Committed by Greg Kroah-Hartman Aug 11, 2023

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



[ Upstream commit 23c0cbd0c75c3b564850294427fd2be2bc2a015b ]

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>
Stable-dep-of: 92e2921eeafd ("ARC: define ASM_NL and __ALIGN(_STR) outside #ifdef __ASSEMBLY__ guard")
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent 5fe86ad6

arch/arc/include/asm/entry-arcv2.h

+0 −8

Original line number	Diff line number	Diff line
		@@ -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

arch/arc/mm/tlbex.S

+11 −0

Original line number	Diff line number	Diff line
		@@ -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