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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id CBB98C27C53 for ; Wed, 12 Jun 2024 08:02:10 +0000 (UTC) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (1024-bit key; unprotected) header.d=citrix.com header.i=@citrix.com header.a=rsa-sha256 header.s=google header.b=H//jupBD; dkim-atps=neutral Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4VzdL85GNvz3fn5 for ; Wed, 12 Jun 2024 18:02:08 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dmarc=pass (p=reject dis=none) header.from=citrix.com Authentication-Results: lists.ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=citrix.com header.i=@citrix.com header.a=rsa-sha256 header.s=google header.b=H//jupBD; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=cloud.com (client-ip=2607:f8b0:4864:20::1133; helo=mail-yw1-x1133.google.com; envelope-from=roger.pau@cloud.com; receiver=lists.ozlabs.org) Received: from mail-yw1-x1133.google.com (mail-yw1-x1133.google.com [IPv6:2607:f8b0:4864:20::1133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4VzdKL4yKJz3cT9 for ; Wed, 12 Jun 2024 18:01:25 +1000 (AEST) Received: by mail-yw1-x1133.google.com with SMTP id 00721157ae682-62cecc3f949so23116117b3.2 for ; Wed, 12 Jun 2024 01:01:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=citrix.com; s=google; t=1718179283; x=1718784083; darn=lists.ozlabs.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=1PGf6HlJc1ANQenmlixWu1Sno9FA8D3ndcnX4NZVy9I=; b=H//jupBDXRkDu2/XGyWMzRZn+gahVPI9HbtLUz56Ffp+RM3uBWL6t+AIUTFQj/BAcm vzl3FRiNuRGGmgdpe69sBYqbSVaW6kI/Hw85aKZD7LJ+61srkHL5rXgk0bE19Iv8m687 wP/Uyt9PHQQd3QsS6gZtT51/RczHSbMJVvUc0= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1718179283; x=1718784083; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=1PGf6HlJc1ANQenmlixWu1Sno9FA8D3ndcnX4NZVy9I=; b=J7tJsuHKq9zaJcA6OXKyjbGJ+gZhRk+gBQjZPw6IJl/DyoGD9mYoxjKXhqcyLFZKqY NZ5K7BiYZoYGFBOUgtOGadZGR8Pf4ybj4ZF4/Q6/GeVRvNUAdG0veIA0PJ+qd8PU1kKk jXODcTZopLLqT7LmHr91u4sYBHsrspiCjR4qGxsabNr3t8tbPa9S6HkwBarKbASVD3J8 4PPGSqqUnGRqG5aUGQR9F6pXqKm4NYyQGgL4GbPbTvACD72J3lSEf5GHMCs1KwiBc71B UdBhc7M5kCL2AC9kItl540b/9o63VFKWNkWZE44GwXmYyJedxKhrJAYHknCkGdciM4Pr anOw== X-Forwarded-Encrypted: i=1; AJvYcCXcDkl8BUACaY2FbkIfB1JEi4nGPz3bHgw7fN7xkbp/iMcpXuwg6LTf4wL5+brDBKZUGeXyi991CrW89B0fPlxcZk1+9YnuuWC/0pqXKw== X-Gm-Message-State: AOJu0YzT86ZlNcol3ghJPZ5KJAbTR7uXPHlXSxGmIjsWvQRaKpnhbujA BqdT+nBz+qIlwFPGwGr/hedPe/4Ak/nJ7HpjsUHHrGwdercoa4wju+1Rv+W2xNY= X-Google-Smtp-Source: AGHT+IEk5bObzz8qCZsqdRqFNOU7a/dh67daOPgnEKJGJzh0sP06pKqPApTvT03CHGJxr8rf1zPYww== X-Received: by 2002:a81:b647:0:b0:61b:e62e:73f1 with SMTP id 00721157ae682-62fb8a58273mr12605907b3.3.1718179282688; Wed, 12 Jun 2024 01:01:22 -0700 (PDT) Received: from localhost ([46.222.2.38]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6b093aff889sm6894416d6.101.2024.06.12.01.01.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 12 Jun 2024 01:01:22 -0700 (PDT) Date: Wed, 12 Jun 2024 10:01:18 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Christoph Hellwig Subject: Re: [PATCH 10/26] xen-blkfront: don't disable cache flushes when they fail Message-ID: References: <20240611051929.513387-1-hch@lst.de> <20240611051929.513387-11-hch@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20240611051929.513387-11-hch@lst.de> X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: nvdimm@lists.linux.dev, "Michael S. Tsirkin" , Jason Wang , linux-nvme@lists.infradead.org, Song Liu , linux-mtd@lists.infradead.org, Vineeth Vijayan , linux-bcache@vger.kernel.org, Alasdair Kergon , drbd-dev@lists.linbit.com, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, Richard Weinberger , Geert Uytterhoeven , Yu Kuai , dm-devel@lists.linux.dev, linux-um@lists.infradead.org, Mike Snitzer , Josef Bacik , Ming Lei , linux-raid@vger.kernel.org, linux-m68k@lists.linux-m68k.org, Mikulas Patocka , xen-devel@lists.xenproject.org, ceph-devel@vger.kernel.org, nbd@other.debian.org, Jens Axboe , linux-block@vger.kernel.org, "Martin K. Petersen" , linux-mmc@vger.kernel.org, Philipp Reisner , Christoph =?utf-8?Q?B=C3=B6hmwalder?= , virtualization@lists.linux.dev, Lars Ellenberg , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Jun 11, 2024 at 07:19:10AM +0200, Christoph Hellwig wrote: > blkfront always had a robust negotiation protocol for detecting a write > cache. Stop simply disabling cache flushes when they fail as that is > a grave error. It's my understanding the current code attempts to cover up for the lack of guarantees the feature itself provides: * feature-barrier * Values: 0/1 (boolean) * Default Value: 0 * * A value of "1" indicates that the backend can process requests * containing the BLKIF_OP_WRITE_BARRIER request opcode. Requests * of this type may still be returned at any time with the * BLKIF_RSP_EOPNOTSUPP result code. * * feature-flush-cache * Values: 0/1 (boolean) * Default Value: 0 * * A value of "1" indicates that the backend can process requests * containing the BLKIF_OP_FLUSH_DISKCACHE request opcode. Requests * of this type may still be returned at any time with the * BLKIF_RSP_EOPNOTSUPP result code. So even when the feature is exposed, the backend might return EOPNOTSUPP for the flush/barrier operations. Such failure is tied on whether the underlying blkback storage supports REQ_OP_WRITE with REQ_PREFLUSH operation. blkback will expose "feature-barrier" and/or "feature-flush-cache" without knowing whether the underlying backend supports those operations, hence the weird fallback in blkfront. I'm unsure whether lack of REQ_PREFLUSH support is not something that we should worry about, it seems like it was when the code was introduced, but that's > 10y ago. Overall blkback should ensure that REQ_PREFLUSH is supported before exposing "feature-barrier" or "feature-flush-cache", as then the exposed features would really match what the underlying backend supports (rather than the commands blkback knows about). Thanks, Roger.