From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id 9BAE12C00A2 for ; Thu, 24 Oct 2013 20:55:15 +1100 (EST) Subject: Re: [PATCH] [RFC] Emulate "lwsync" to run standard user land on e500 cores Mime-Version: 1.0 (Apple Message framework v1283) Content-Type: text/plain; charset=us-ascii From: Kumar Gala In-Reply-To: <1382607919.9395.56.camel@pasglop> Date: Thu, 24 Oct 2013 04:55:02 -0500 Message-Id: <6BFC8EB0-1A75-41C3-985A-E3ED14846710@kernel.crashing.org> References: <1382081880-6666-1-git-send-email-wd@denx.de> <39CCEB38-1D9B-4918-B8F4-148D4E90FE21@kernel.crashing.org> <1382523314.3926.21.camel@aoeu.buserror.net> <1382607919.9395.56.camel@pasglop> To: Benjamin Herrenschmidt Cc: Scott Wood , linuxppc-dev@lists.ozlabs.org, Wolfgang Denk List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Oct 24, 2013, at 4:45 AM, Benjamin Herrenschmidt wrote: > On Wed, 2013-10-23 at 23:06 -0500, Kumar Gala wrote: >> On Oct 23, 2013, at 5:15 AM, Scott Wood wrote: >>=20 >>> On Wed, 2013-10-23 at 00:07 -0500, Kumar Gala wrote: >>>> On Oct 18, 2013, at 2:38 AM, Wolfgang Denk wrote: >>>>> diff --git a/arch/powerpc/kernel/traps.c = b/arch/powerpc/kernel/traps.c >>>>> index f783c93..f330374 100644 >>>>> --- a/arch/powerpc/kernel/traps.c >>>>> +++ b/arch/powerpc/kernel/traps.c >>>>> @@ -986,6 +986,13 @@ static int emulate_instruction(struct pt_regs = *regs) >>>>> return 0; >>>>> } >>>>>=20 >>>>> + /* Emulating the lwsync insn as a sync insn */ >>>>> + if (instword =3D=3D PPC_INST_LWSYNC) { >>>>> + PPC_WARN_EMULATED(lwsync, regs); >>>>> + asm volatile("sync" : : : "memory"); >>>>=20 >>>> Do we really need the inline asm? Doesn't the fact of just taking = an exception and returning from it equate to a sync. >>>=20 >>> No, it doesn't equate to a sync. See the discussion here: >>> http://patchwork.ozlabs.org/patch/256747/ >>>=20 >>=20 >> Thanks.=20 >>=20 >> I'm not sure I'm a fan of doing this as it silently hides a = significant performance impact. >>=20 >> Could we possible re-write the userspace instruction to be a 'sync' = when we hit this? >=20 > Rewriting user space is a can of worms I wouldn't get into ... is any > other arch doing it ? Fair enough >=20 > I'm not too worried as long as we warn and account them. Than, I'd ask this be under a Kconfig option that is disabled by = default. Users should have to explicitly enable this so they know what = they are doing. - k