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 13170C4332F for ; Wed, 9 Nov 2022 15:53:11 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1osnN1-0008Gb-Th; Wed, 09 Nov 2022 10:52:07 -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 1osnMz-0008Fz-Ie for qemu-devel@nongnu.org; Wed, 09 Nov 2022 10:52:05 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1osnMx-0000i9-Sl for qemu-devel@nongnu.org; Wed, 09 Nov 2022 10:52:05 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1668009123; h=from:from:reply-to: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=z3AuM9E3/sI/N7pHpXKop6sn0N1f3MNeW7M7Zxz51P4=; b=X5XkrawPwHCYzy8CSBMCpXlxYhTfKTmH2ALyCTkWYXBj60p/cG8toyE7n5iyoRRpKJuIJd ygRk50oKdkLjD1PNlWOvv+VZgyxh2N3Fyrsma10rUKVo976fSOXlGS/F8iEG387vpHTUSh AAbsomMtRWIzUf5v7dc73bSW/CPjwKU= 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-632-VRGZY0DsNeCys9MndPoAJQ-1; Wed, 09 Nov 2022 10:51:59 -0500 X-MC-Unique: VRGZY0DsNeCys9MndPoAJQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 3C13B86F12E; Wed, 9 Nov 2022 15:51:59 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.46]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 779DD2024CC6; Wed, 9 Nov 2022 15:51:57 +0000 (UTC) Date: Wed, 9 Nov 2022 15:51:55 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Markus Armbruster 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 Message-ID: References: <20221107130217.2243815-1-sunilvl@ventanamicro.com> <871qqehib4.fsf@linaro.org> <20221107173201.343hkqqugkzdzqcf@kamzik> <87y1skkv2a.fsf@pond.sub.org> <878rkkku7l.fsf@pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <878rkkku7l.fsf@pond.sub.org> User-Agent: Mutt/2.2.7 (2022-08-07) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@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=unavailable 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: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Nov 09, 2022 at 04:45:18PM +0100, Markus Armbruster wrote: > Daniel P. Berrangé writes: > > > On Wed, Nov 09, 2022 at 04:26:53PM +0100, Markus Armbruster wrote: > >> Daniel P. Berrangé 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 some > >> >> 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. > >> > >> 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? > > > > Is (C) actually a separate issue ? I thought it was simply the > > result of (B) ? ie if we skip reading the zero padding, we won't > > be dirtying the memory with lots of zeros. we'll have mmap'd the > > full 64 MB, but most won't be paged in since we wouldn't write > > the zeros to it. Only if the guest writes to those areas do we > > need to then flush it back out. > > I expressed myself poorly. All three are related, but there's still a > distinction between each of them in my thinking. > > Say we use an image format that compresses data. Represents the padding > efficiently. Storage on disk is efficient (A), and so is reading it > (B). Trouble is decompressing it to memory dirties the memory unless we > take care not to write all-zero pages (C). > > Clearer now? Ok yeah, so reading can be efficient, but if the reader doesn't pay attention to where the holes are, it'll dirty memory anyway. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|