From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (ozlabs.org [103.22.144.67]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3D99F1A0018 for ; Fri, 20 Nov 2015 22:01:11 +1100 (AEDT) Message-ID: <1448017264.25731.1.camel@ellerman.id.au> Subject: Re: [PATCH 5/8] powerpc: Restore FPU/VEC/VSX if previously used From: Michael Ellerman To: Cyril Bur , mikey@neuling.org, anton@samba.org, linuxppc-dev@ozlabs.org Date: Fri, 20 Nov 2015 22:01:04 +1100 In-Reply-To: <1447817215-15804-6-git-send-email-cyrilbur@gmail.com> References: <1447817215-15804-1-git-send-email-cyrilbur@gmail.com> <1447817215-15804-6-git-send-email-cyrilbur@gmail.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, 2015-11-18 at 14:26 +1100, Cyril Bur wrote: > diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S > index c8b4225..46e9869 100644 > --- a/arch/powerpc/kernel/entry_64.S > +++ b/arch/powerpc/kernel/entry_64.S > @@ -210,7 +210,54 @@ system_call: /* label this so stack traces look sane */ > li r11,-MAX_ERRNO > andi. r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK) > bne- syscall_exit_work > - cmpld r3,r11 > + > + /* > + * This is an assembly version of checks performed in restore_math() > + * to avoid calling C unless absolutely necessary. > + * Note: In order to simplify the assembly, if the FP or VEC registers > + * are hot (and therefore restore_math() isn't called) the > + * LOAD_{FP,VEC} thread counter doesn't get incremented. > + * This is likely the best thing to do anyway because hot regs indicate > + * that the workload is doing a lot of syscalls that can be handled > + * quickly and without the need to touch FP or VEC regs (by the kernel). > + * a) If this workload is long running then this is exactly what the > + * kernel should be doing. > + * b) If this workload isn't long running then we'll soon fall back to > + * calling into C and the counter will be incremented regularly again > + * anyway. > + */ > + ld r9,PACACURRENT(r13) > + andi. r0,r8,MSR_FP > + addi r9,r9,THREAD > + lbz r5,THREAD_LOAD_FP(r9) > + /* > + * Goto 2 if !r0 && r5 > + * The cmpb works because r5 can only have bits set in the lowest byte > + * and r0 may or may not have bit 13 set (different byte) but will have > + * a zero low byte therefore the low bytes must differ if r5 == true > + * and the bit 13 byte must be the same if !r0 > + */ > + cmpb r7,r0,r5 cmpb is new since Power6, which means it doesn't exist on Cell -> Program Check :) I'm testing a patch using crandc, but I don't like it. I'm not a big fan of the logic here, it's unpleasantly complicated. Did you benchmark going to C to do the checks? Or I wonder if we could just check THREAD_LOAD_FP || THREAD_LOAD_VEC and if either is set we go to restore_math(). Or on the other hand we check !MSR_FP && !MSR_VEC and if so we go to restore_math()? > + cmpldi r7,0xff0 > +#ifdef CONFIG_ALTIVEC > + beq 2f > + > + lbz r9,THREAD_LOAD_VEC(r9) > + andis. r0,r8,MSR_VEC@h > + /* Skip (goto 3) if r0 || !r9 */ > + bne 3f > + cmpldi r9,0 > + beq 3f > +#else > + bne 3f > +#endif > +2: addi r3,r1,STACK_FRAME_OVERHEAD > + bl restore_math > + ld r8,_MSR(r1) > + ld r3,RESULT(r1) > + li r11,-MAX_ERRNO > + > +3: cmpld r3,r11 > ld r5,_CCR(r1) > bge- syscall_error > .Lsyscall_error_cont: cheers