From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:51644) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hH9Xb-0007Z6-Sd for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:05:37 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hH9Xa-0004Vu-BS for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:05:35 -0400 Received: from mail-oi1-x243.google.com ([2607:f8b0:4864:20::243]:40889) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1hH9XZ-0004Ht-Uu for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:05:34 -0400 Received: by mail-oi1-x243.google.com with SMTP id f196so2005835oib.7 for ; Thu, 18 Apr 2019 09:05:18 -0700 (PDT) MIME-Version: 1.0 References: <20190410040826.24371-1-pagupta@redhat.com> <20190410040826.24371-2-pagupta@redhat.com> <20190412083230.GA29850@quack2.suse.cz> In-Reply-To: From: Dan Williams Date: Thu, 18 Apr 2019 09:05:05 -0700 Message-ID: Content-Type: text/plain; charset="UTF-8" Subject: Re: [Qemu-devel] [PATCH v5 1/6] libnvdimm: nd_region flush callback support List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jeff Moyer Cc: Jan Kara , Pankaj Gupta , linux-nvdimm , Linux Kernel Mailing List , virtualization@lists.linux-foundation.org, KVM list , linux-fsdevel , Linux ACPI , Qemu Developers , linux-ext4 , linux-xfs , Ross Zwisler , Vishal L Verma , Dave Jiang , "Michael S. Tsirkin" , Jason Wang , Matthew Wilcox , "Rafael J. Wysocki" , Christoph Hellwig , Len Brown , Theodore Ts'o , Andreas Dilger , "Darrick J. Wong" , lcapitulino@redhat.com, Kevin Wolf , Igor Mammedov , Nitesh Narayan Lal , Rik van Riel , Stefan Hajnoczi , Andrea Arcangeli , David Hildenbrand , david , cohuck@redhat.com, Xiao Guangrong , Paolo Bonzini , kilobyte@angband.pl, yuval shaia On Fri, Apr 12, 2019 at 6:12 AM Jeff Moyer wrote: > > Jan Kara writes: > > > On Thu 11-04-19 07:51:48, Dan Williams wrote: > >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta wrote: > >> > + } else { > >> > + if (nd_region->flush(nd_region)) > >> > + rc = -EIO; > >> > >> Given the common case wants to be fast and synchronous I think we > >> should try to avoid retpoline overhead by default. So something like > >> this: > >> > >> if (nd_region->flush == generic_nvdimm_flush) > >> rc = generic_nvdimm_flush(...); > > > > I'd either add a comment about avoiding retpoline overhead here or just > > make ->flush == NULL mean generic_nvdimm_flush(). Just so that people don't > > get confused by the code. > > Isn't this premature optimization? I really don't like adding things > like this without some numbers to show it's worth it. I don't think it's premature given this optimization technique is already being deployed elsewhere, see: https://lwn.net/Articles/774347/ 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 X-Spam-Level: X-Spam-Status: No, score=-0.7 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 56985C10F0E for ; Thu, 18 Apr 2019 16:06:26 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 17CF321479 for ; Thu, 18 Apr 2019 16:06:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=intel-com.20150623.gappssmtp.com header.i=@intel-com.20150623.gappssmtp.com header.b="ATWDxkO+" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 17CF321479 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([127.0.0.1]:43785 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hH9YP-0007sA-Dm for qemu-devel@archiver.kernel.org; Thu, 18 Apr 2019 12:06:25 -0400 Received: from eggs.gnu.org ([209.51.188.92]:51644) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hH9Xb-0007Z6-Sd for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:05:37 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hH9Xa-0004Vu-BS for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:05:35 -0400 Received: from mail-oi1-x243.google.com ([2607:f8b0:4864:20::243]:40889) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.71) (envelope-from ) id 1hH9XZ-0004Ht-Uu for qemu-devel@nongnu.org; Thu, 18 Apr 2019 12:05:34 -0400 Received: by mail-oi1-x243.google.com with SMTP id f196so2005835oib.7 for ; Thu, 18 Apr 2019 09:05:18 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=intel-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=9656FNKKp4cUVga3XUcreKMqWUBjOMnkCExkfsBvcE0=; b=ATWDxkO+EzohN5YWedA5OnfOl7z0C5wosAmTrDF5hxxtYtjNdCK/9PHgnZhYETJJ8t vacr+rHkJxxNGOmeI1vTif1Vl4SIwdCSz21szdtTalMtQ8a/1Obxe+VdKwZnY8xrFtd+ ciBIJxCm9FZlAQVPiOTKZr8Xdghbnp/hQTXS/ucudKTmB79U11kjETh4fqNlfW3T1gK0 Zu87he2RHQTC+AM19urSAzmn3PITVUqT2oXHTybHWlpO7u1bFfMxyXZLHVTzCvZ6mXjc bD19cp4gsnL0R2s/7Y3bFqcAI2a5/xvDNGJEJYE3ed3iSmhC+ui2TozfrYHnQqDdah3m ZD3A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=9656FNKKp4cUVga3XUcreKMqWUBjOMnkCExkfsBvcE0=; b=oGbgiyewMExVmzhXa4aRoKzdMd0XIoCJkwyUlh/R2uZhRzQAeSRdP0GULwrgs46eYr Y8DDunm4iZvG3Ry/7/i+cCw01E1E8e1dZJdFFB8BnR3RcUnprbZxAbCalsLqW5Lx9o4g tybikKIGyL1i5DVjswc/yNm0W0Rx40W75iW4QMAi62XONCbIsSdCgXsWxiYpoDSQk1+o KEcsovVQ2EAPB2632GTWjop6b+EZL633edmSRV6RtqIrHtQPPcbA0Wrjz1IHnWaxW/HC 8VYWaF/3mjYPW4hBEpeSvxX60fcG74x+WbfmElIH7VRzYxhXsBNy8JRuNj1+C+YuTGwM 8o8A== X-Gm-Message-State: APjAAAX/Zi3DecZDXWM8vJB7jevwSSISfyTGzTPh7eQ+o/c5+6um5XRp bHTWGS8y4WmYgFMo1Iu3Kem3Hj2Bp0+l8uGWlPFbew== X-Google-Smtp-Source: APXvYqzTGVz9ejtUV0liyJEyYqaIHrUyuHEbCYPG5pgMNrA7E3qjZWVF7XUM3SvNiTSOlLSiE2pi4l5w04d9dnUuvxA= X-Received: by 2002:aca:ed88:: with SMTP id l130mr2422676oih.70.1555603517276; Thu, 18 Apr 2019 09:05:17 -0700 (PDT) MIME-Version: 1.0 References: <20190410040826.24371-1-pagupta@redhat.com> <20190410040826.24371-2-pagupta@redhat.com> <20190412083230.GA29850@quack2.suse.cz> In-Reply-To: From: Dan Williams Date: Thu, 18 Apr 2019 09:05:05 -0700 Message-ID: To: Jeff Moyer Content-Type: text/plain; charset="UTF-8" X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 2607:f8b0:4864:20::243 Subject: Re: [Qemu-devel] [PATCH v5 1/6] libnvdimm: nd_region flush callback support X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Pankaj Gupta , cohuck@redhat.com, Jan Kara , KVM list , "Michael S. Tsirkin" , Jason Wang , david , Qemu Developers , virtualization@lists.linux-foundation.org, Andreas Dilger , Ross Zwisler , Andrea Arcangeli , Dave Jiang , linux-nvdimm , Vishal L Verma , David Hildenbrand , Matthew Wilcox , Christoph Hellwig , Linux ACPI , linux-ext4 , Len Brown , kilobyte@angband.pl, Rik van Riel , yuval shaia , Stefan Hajnoczi , Paolo Bonzini , lcapitulino@redhat.com, Kevin Wolf , Nitesh Narayan Lal , Theodore Ts'o , Xiao Guangrong , "Darrick J. Wong" , "Rafael J. Wysocki" , Linux Kernel Mailing List , linux-xfs , linux-fsdevel , Igor Mammedov Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Message-ID: <20190418160505.wJI5qaiqpt2ngX1T_nzzA-SzY0rDe1ajUvKTp8OlkNg@z> On Fri, Apr 12, 2019 at 6:12 AM Jeff Moyer wrote: > > Jan Kara writes: > > > On Thu 11-04-19 07:51:48, Dan Williams wrote: > >> On Tue, Apr 9, 2019 at 9:09 PM Pankaj Gupta wrote: > >> > + } else { > >> > + if (nd_region->flush(nd_region)) > >> > + rc = -EIO; > >> > >> Given the common case wants to be fast and synchronous I think we > >> should try to avoid retpoline overhead by default. So something like > >> this: > >> > >> if (nd_region->flush == generic_nvdimm_flush) > >> rc = generic_nvdimm_flush(...); > > > > I'd either add a comment about avoiding retpoline overhead here or just > > make ->flush == NULL mean generic_nvdimm_flush(). Just so that people don't > > get confused by the code. > > Isn't this premature optimization? I really don't like adding things > like this without some numbers to show it's worth it. I don't think it's premature given this optimization technique is already being deployed elsewhere, see: https://lwn.net/Articles/774347/