Re: [RFC v2 10/12] powerpc: Read AMR only if pkey-violation caused the exception.
From: Ram Pai
Date: Tue Jun 20 2017 - 19:59:16 EST
On Tue, Jun 20, 2017 at 12:16:40PM +0530, Anshuman Khandual wrote:
> On 06/19/2017 11:29 PM, Ram Pai wrote:
> > On Mon, Jun 19, 2017 at 09:06:13PM +1000, Michael Ellerman wrote:
> >> Ram Pai <linuxram@xxxxxxxxxx> writes:
> >>> Signed-off-by: Ram Pai <linuxram@xxxxxxxxxx>
> >>> ---
> >>> arch/powerpc/kernel/exceptions-64s.S | 16 ++++++++++------
> >>> 1 file changed, 10 insertions(+), 6 deletions(-)
> >>> diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
> >>> index 8db9ef8..a4de1b4 100644
> >>> --- a/arch/powerpc/kernel/exceptions-64s.S
> >>> +++ b/arch/powerpc/kernel/exceptions-64s.S
> >>> @@ -493,13 +493,15 @@ EXC_COMMON_BEGIN(data_access_common)
> >>> ld r12,_MSR(r1)
> >>> ld r3,PACA_EXGEN+EX_DAR(r13)
> >>> lwz r4,PACA_EXGEN+EX_DSISR(r13)
> >>> + std r3,_DAR(r1)
> >>> + std r4,_DSISR(r1)
> >>> #ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
> >>> + andis. r0,r4,DSISR_KEYFAULT@h /* save AMR only if its a key fault */
> >>> + beq+ 1f
> >> This seems to be incremental on top of one of your other patches.
> >> But I don't see why, can you please just squash this into whatever patch
> >> adds this code in the first place.
> > It was an optimization added later. But yes it can be squashed into an
> > earlier patch.
> Could you please explain what is the optimization this achieves ?
Don't want to read the AMR if it is not a key protection fault. This is a hot-hot-path.
A few cycles saved can accumulate into signficant savings overall.