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=-0.9 required=3.0 tests=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 B19FEC04AAD for ; Wed, 8 May 2019 13:21:32 +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 89EB1214AF for ; Wed, 8 May 2019 13:21:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 89EB1214AF Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.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]:37109 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hOMVn-0003MC-OJ for qemu-devel@archiver.kernel.org; Wed, 08 May 2019 09:21:31 -0400 Received: from eggs.gnu.org ([209.51.188.92]:34448) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1hOMUf-0002jW-Ln for qemu-devel@nongnu.org; Wed, 08 May 2019 09:20:22 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hOMUd-00075I-Tm for qemu-devel@nongnu.org; Wed, 08 May 2019 09:20:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:39988) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hOMUb-0006r9-C9; Wed, 08 May 2019 09:20:17 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D508A30842B2; Wed, 8 May 2019 13:20:15 +0000 (UTC) Received: from blackfin.pond.sub.org (ovpn-116-28.ams2.redhat.com [10.36.116.28]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 517241724D; Wed, 8 May 2019 13:20:13 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id A2B401132B35; Wed, 8 May 2019 15:20:10 +0200 (CEST) From: Markus Armbruster To: Laszlo Ersek References: <20190505070059.4664-1-zhengxiang9@huawei.com> <87a7fyb0v7.fsf@dusky.pond.sub.org> <72adbed8-f650-42df-98d5-e98154baec08@redhat.com> Date: Wed, 08 May 2019 15:20:10 +0200 In-Reply-To: <72adbed8-f650-42df-98d5-e98154baec08@redhat.com> (Laszlo Ersek's message of "Tue, 7 May 2019 21:03:51 +0200") Message-ID: <87h8a513sl.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.40]); Wed, 08 May 2019 13:20:15 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH] pflash: Only read non-zero parts of backend image 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: kwolf@redhat.com, peter.maydell@linaro.org, qemu-block@nongnu.org, ard.biesheuvel@linaro.org, qemu-devel@nongnu.org, mreitz@redhat.com, Xiang Zheng , stefanha@redhat.com, guoheyi@huawei.com, wanghaibin.wang@huawei.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Laszlo Ersek writes: > Hi Markus, > > On 05/07/19 20:01, Markus Armbruster wrote: >> The subject is slightly misleading. Holes read as zero. So do >> non-holes full of zeroes. The patch avoids reading the former, but >> still reads the latter. >> >> Xiang Zheng writes: >> >>> Currently we fill the memory space with two 64MB NOR images when >>> using persistent UEFI variables on virt board. Actually we only use >>> a very small(non-zero) part of the memory while the rest significant >>> large(zero) part of memory is wasted. >> >> Neglects to mention that the "virt board" is ARM. >> >>> So this patch checks the block status and only writes the non-zero part >>> into memory. This requires pflash devices to use sparse files for >>> backends. >> >> I started to draft an improved commit message, but then I realized this >> patch can't work. >> >> The pflash_cfi01 device allocates its device memory like this: >> >> memory_region_init_rom_device( >> &pfl->mem, OBJECT(dev), >> &pflash_cfi01_ops, >> pfl, >> pfl->name, total_len, &local_err); >> >> pflash_cfi02 is similar. >> >> memory_region_init_rom_device() calls >> memory_region_init_rom_device_nomigrate() calls qemu_ram_alloc() calls >> qemu_ram_alloc_internal() calls g_malloc0(). Thus, all the device >> memory gets written to even with this patch. > > As far as I can see, qemu_ram_alloc_internal() calls g_malloc0() only to > allocate the the new RAMBlock object called "new_block". The actual > guest RAM allocation occurs inside ram_block_add(), which is also called > by qemu_ram_alloc_internal(). You're right. I should've read more attentively. > One frame outwards the stack, qemu_ram_alloc() passes NULL to > qemu_ram_alloc_internal(), for the 4th ("host") parameter. Therefore, in > qemu_ram_alloc_internal(), we set "new_block->host" to NULL as well. > > Then in ram_block_add(), we take the (!new_block->host) branch, and call > phys_mem_alloc(). > > Unfortunately, "phys_mem_alloc" is a function pointer, set with > phys_mem_set_alloc(). The phys_mem_set_alloc() function is called from > "target/s390x/kvm.c" (setting the function pointer to > legacy_s390_alloc()), so it doesn't apply in this case. Therefore we end > up calling the default qemu_anon_ram_alloc() function, through the > funcptr. (I think anyway.) > > And qemu_anon_ram_alloc() boils down to mmap() + MAP_ANONYMOUS, in > qemu_ram_mmap(). (Even on PPC64 hosts, because qemu_anon_ram_alloc() > passes (-1) for "fd".) > > I may have missed something, of course -- I obviously didn't test it, > just speculated from the source. Thanks for your sleuthing! >> I'm afraid you neglected to test. Accusation actually unsupported. I apologize, and replace it by a question: have you observed the improvement you're trying to achieve, and if yes, how? [...]