From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (ozlabs.org [IPv6:2401:3900:2:1::2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id B201A1A000C for ; Mon, 23 Nov 2015 10:07:13 +1100 (AEDT) Message-ID: <1448233633.15853.1.camel@ellerman.id.au> Subject: Re: [PATCH 5/8] powerpc: Restore FPU/VEC/VSX if previously used From: Michael Ellerman To: Cyril Bur Cc: mikey@neuling.org, anton@samba.org, linuxppc-dev@ozlabs.org Date: Mon, 23 Nov 2015 10:07:13 +1100 In-Reply-To: <20151123091818.576d92af@camb691> References: <1447817215-15804-1-git-send-email-cyrilbur@gmail.com> <1447817215-15804-6-git-send-email-cyrilbur@gmail.com> <1448017264.25731.1.camel@ellerman.id.au> <20151123091818.576d92af@camb691> 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 Mon, 2015-11-23 at 09:18 +1100, Cyril Bur wrote: > On Fri, 20 Nov 2015 22:01:04 +1100 > Michael Ellerman wrote: > > 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 :) > > > Oops, sorry. That's fine, there's almost no way for you to know that from reading the documentation. > > 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(). > > > > I didn't benchmark going to C mostly because you wanted to avoid calling C > unless necessary in that path. Based off the results I got benchmarking the > this series I expect calling C will also be in the noise of removing the > exception. Yeah I figured it was probably me that said "avoid C at all costs". But I've changed my mind ;) > > Or on the other hand we check !MSR_FP && !MSR_VEC and if so we go to > > restore_math()? > > That seems like the best check to leave in the assembly if you want to avoid > complicated assembly in there. Cool. If you can benchmark that that'd be great, mmkay. cheers