From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0C095C433EF for ; Tue, 15 Mar 2022 11:22:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347582AbiCOLYE (ORCPT ); Tue, 15 Mar 2022 07:24:04 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39570 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237382AbiCOLYA (ORCPT ); Tue, 15 Mar 2022 07:24:00 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B6EBB631F; Tue, 15 Mar 2022 04:22:47 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=Ul9b8YQaPGFJIzLg4O7EZ8BuJh4nEAPn8uLCo67QHe0=; b=Q7xpYHxRR2cIPtVMvd+GXrt1E4 M+7WVfOhHTLT7CrXI6JZTs2mSi2JnYwSgR5RGTK9odO9Tla4PsLVnjQYvn9GLdqDZ8ApxtXjXXQsH giIuzg4sZ4Bsl8k5WEb0iMdE/Vw0DQG5fV0R6kdV3WePth0TDG/kt4DADd8xq/nfiqo7MlWjexAu8 dsWzQaOCVIqmIaaFvAx9TKrMmaPQqzh+00txpavVsEKq0EAfSsvveat9LbUOcmsiFhKcUum0bUT3A AEPRAaKQe51KT1GhzQYPyyPEHtDZzXNHi9/ilKhCsqvWLwfhmkmxbCmRt+NcYaeKKjRRGac+CfsIP w649oCkA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=worktop.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1nU5G6-004ytk-K8; Tue, 15 Mar 2022 11:22:34 +0000 Received: by worktop.programming.kicks-ass.net (Postfix, from userid 1000) id C562B98620D; Tue, 15 Mar 2022 12:22:32 +0100 (CET) Date: Tue, 15 Mar 2022 12:22:32 +0100 From: Peter Zijlstra To: Anshuman Khandual Cc: linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, acme@kernel.org, Robin Murphy , Suzuki Poulose , James Clark , Ingo Molnar , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Thomas Gleixner , Will Deacon , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH V4 03/10] perf: Extend branch type classification Message-ID: <20220315112232.GF8939@worktop.programming.kicks-ass.net> References: <20220315053516.431515-1-anshuman.khandual@arm.com> <20220315053516.431515-4-anshuman.khandual@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220315053516.431515-4-anshuman.khandual@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 15, 2022 at 11:05:09AM +0530, Anshuman Khandual wrote: > branch_entry.type now has ran out of space to accommodate more branch types > classification. This will prevent perf branch stack implementation on arm64 > (via BRBE) to capture all available branch types. Extending this bit field > i.e branch_entry.type [4 bits] is not an option as it will break user space > ABI both for little and big endian perf tools. > > Extend branch classification with a new field branch_entry.new_type via a > new branch type PERF_BR_EXTEND_ABI in branch_entry.type. Perf tools which > could decode PERF_BR_EXTEND_ABI, will then parse branch_entry.new_type as > well. > > branch_entry.new_type is a 4 bit field which can hold upto 16 branch types. > The first three branch types will hold various generic page faults followed > by five architecture specific branch types, which can be overridden by the > platform for specific use cases. These architecture specific branch types > gets overridden on arm64 platform for BRBE implementation. > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h > index 26d8f0b5ac0d..d29280adc3c4 100644 > --- a/include/uapi/linux/perf_event.h > +++ b/include/uapi/linux/perf_event.h > @@ -255,9 +255,22 @@ enum { > PERF_BR_IRQ = 12, /* irq */ > PERF_BR_SERROR = 13, /* system error */ > PERF_BR_NO_TX = 14, /* not in transaction */ > + PERF_BR_EXTEND_ABI = 15, /* extend ABI */ > PERF_BR_MAX, > }; > #define PERF_SAMPLE_BRANCH_PLM_ALL \ > (PERF_SAMPLE_BRANCH_USER|\ > PERF_SAMPLE_BRANCH_KERNEL|\ > @@ -1372,7 +1385,8 @@ struct perf_branch_entry { > abort:1, /* transaction abort */ > cycles:16, /* cycle count to last branch */ > type:4, /* branch type */ > - reserved:40; > + new_type:4, /* additional branch type */ > + reserved:36; > }; Hurmpf... this will effectively give us 5 bits of space for the cost of 8, that seems... unfortunate. Would something like: type:4, ext_type:4, reserved:36; and have all software do: type = pbe->type | (pbe->ext_type << 4); Then old software will only know about the old types. New software on old kernels will add 4 0's, which is harmless, while new software on new kernels will get 8 bytes of type. Would that work?