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.gnu.org (lists.gnu.org [209.51.188.17]) (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 79EABC433FE for ; Wed, 9 Nov 2022 15:28:11 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1osmyt-0004Fc-JD; Wed, 09 Nov 2022 10:27:11 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1osmys-0004AJ-Lt for qemu-devel@nongnu.org; Wed, 09 Nov 2022 10:27:10 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1osmyq-00070h-SC for qemu-devel@nongnu.org; Wed, 09 Nov 2022 10:27:10 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1668007626; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=rwqBaj99ui/gUwlHjxV/4pkbk5Oi/YI0YZmoPqu163Y=; b=ZkwE2vSmvtCz/YJ4Ndw92yoMia+S5MiNbMAKGvYgrdgG+0w+CnuMfzG4OaZHdZ3efSX865 D9UPqDGspQ1rUiDduM9RL29gWbMgQ96opyw5A5wR+4HUNKoQH5O+8p5/eLFllZu0wPCamO HMYGQqNrqmCPtVrxLH+032+9+B0RXyA= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-613-3EcaFopoM_27geS5k1Y5kw-1; Wed, 09 Nov 2022 10:27:00 -0500 X-MC-Unique: 3EcaFopoM_27geS5k1Y5kw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id EB9FD101A528; Wed, 9 Nov 2022 15:26:59 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.193.175]) by smtp.corp.redhat.com (Postfix) with ESMTPS id AC902C1908A; Wed, 9 Nov 2022 15:26:58 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 67F3A21E6921; Wed, 9 Nov 2022 16:26:53 +0100 (CET) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: Andrew Jones , Alex =?utf-8?Q?Benn=C3=A9e?= , Sunil V L , Peter Maydell , Palmer Dabbelt , Alistair Francis , Bin Meng , Gerd Hoffmann , qemu-riscv@nongnu.org, qemu-devel@nongnu.org Subject: Re: [PATCH V2] hw/riscv: virt: Remove size restriction for pflash References: <20221107130217.2243815-1-sunilvl@ventanamicro.com> <871qqehib4.fsf@linaro.org> <20221107173201.343hkqqugkzdzqcf@kamzik> Date: Wed, 09 Nov 2022 16:26:53 +0100 In-Reply-To: ("Daniel P. =?utf-8?Q?Berrang?= =?utf-8?Q?=C3=A9=22's?= message of "Mon, 7 Nov 2022 17:34:20 +0000") Message-ID: <87y1skkv2a.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.8 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Daniel P. Berrang=C3=A9 writes: > On Mon, Nov 07, 2022 at 06:32:01PM +0100, Andrew Jones wrote: [...] >> Padding is a good idea, but too much causes other problems. When building >> lightweight VMs which may pull the firmware image from a network, >> AArch64 VMs require 64MB of mostly zeros to be transferred first, which >> can become a substantial amount of the overall boot time[*]. Being able = to >> create images smaller than the total flash device size, but still add so= me >> pad for later growth, seems like the happy-medium to shoot for. > > QEMU configures the firmware using -blockdev, Yes, even though the devices in question are not block devices. > so can use any file > format that QEMU supports at the block layer. IOW, you can store > the firmware in a qcow2 file and thus you will never fetch any > of the padding zeros to be transferred. That said I'm not sure > that libvirt supports anything other than a raw file today.=20 Here's another idea. The "raw" format supports exposing a slice of the underlying block node (options @offset and @size). It could support padding. Writing to the padding should then grow the underlying node. Taking a step back to look at the bigger picture... there are three issues, I think: (A) Storing padding on disk is wasteful. Use a file system that supports sparse files, or an image format that can represent the padding efficiently. (B) Reading padding into memory is wasteful. Matters mostly when a network is involved. Use an image format that can represent the padding efficiently. (C) Dirtying memory for padding is wasteful. I figure KSM could turn zero-padding into holes. We could play with mmap() & friends. Other ideas? Any solution needs to work both for read-only and read/write padding. Throwing away data written to the padding on cold restart is not what I'd regard as "works".