From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753574AbbCYSTb (ORCPT ); Wed, 25 Mar 2015 14:19:31 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35555 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752187AbbCYST3 (ORCPT ); Wed, 25 Mar 2015 14:19:29 -0400 Message-ID: <5512FC15.30209@redhat.com> Date: Wed, 25 Mar 2015 19:19:01 +0100 From: Denys Vlasenko User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0 MIME-Version: 1.0 To: Ingo Molnar CC: Linus Torvalds , Steven Rostedt , Borislav Petkov , "H. Peter Anvin" , Andy Lutomirski , Oleg Nesterov , Frederic Weisbecker , Alexei Starovoitov , Will Drewry , Kees Cook , x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64 path References: <1427303896-24023-1-git-send-email-dvlasenk@redhat.com> <1427303896-24023-2-git-send-email-dvlasenk@redhat.com> <20150325172922.GA7321@gmail.com> <5512F339.8060209@redhat.com> <20150325180441.GB7321@gmail.com> In-Reply-To: <20150325180441.GB7321@gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/25/2015 07:04 PM, Ingo Molnar wrote: >>> * Denys Vlasenko wrote: >>> >>>> SYSRET code path has a small irq-off block. >>>> On this code path, TRACE_IRQS_ON can't be called right before interrupts >>>> are enabled for real, we can't clobber registers there. >>>> So current code does it earlier, in a safe place. >>>> >>>> But with this, TRACE_IRQS_OFF/ON frames just two fast instructions, >>>> which is ridiculous: now most of irq-off block is _outside_ of the framing. >>>> >>>> Do the same thing that we do on SYSCALL entry: do not track this irq-off block, >>>> it is very small to ever cause noticeable irq latency. >>>> >>>> Be careful: make sure that "jnz int_ret_from_sys_call_irqs_off" now does >>>> invoke TRACE_IRQS_OFF - move int_ret_from_sys_call_irqs_off label before >>>> TRACE_IRQS_OFF. >>> >>>> @@ -345,8 +346,8 @@ tracesys_phase2: >>>> */ >>>> GLOBAL(int_ret_from_sys_call) >>>> DISABLE_INTERRUPTS(CLBR_NONE) >>>> - TRACE_IRQS_OFF >>>> int_ret_from_sys_call_irqs_off: >>>> + TRACE_IRQS_OFF >>>> movl $_TIF_ALLWORK_MASK,%edi >>>> /* edi: mask to check */ >>> >>> This latter trick absolutely needs a comment, to keep future lockdep >>> developers from wondering about the mismatch and the weird label >>> placement ... >> >> Unsure how to format it. >> >> How about: >> >> >> DISABLE_INTERRUPTS(CLBR_NONE) >> int_ret_from_sys_call_irqs_off: /* jumps come here with irqs off */ >> TRACE_IRQS_OFF > > Why not something like 'jumps come here from the irqs-off SYSRET > path'? Ok. I'll send v2 for patches 1 and 2. (Patch 1 will be expanded, there is another instance of jump needing the same treatment.) >> >> >> >> (In truth, there is only one jump as of now, but using pliral >> "jumps" if that would change) > > I'd also put a comment to the actual sysret IRQ-disablement that we > are skipping with the annotation. Patch does add such a comment: + /* + * We do not frame this tiny irq-off block with TRACE_IRQS_OFF/ON, + * it is too small to ever cause noticeable irq latency. + */