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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 ([::1]:57024 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kVJBM-0005J8-KG for qemu-devel@archiver.kernel.org; Wed, 21 Oct 2020 14:49:56 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:41978) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kVJAn-0004tp-8Z for qemu-devel@nongnu.org; Wed, 21 Oct 2020 14:49:21 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:48429) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kVJAj-0007pN-GS for qemu-devel@nongnu.org; Wed, 21 Oct 2020 14:49:20 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1603306154; 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=v9mBWl8fBOtUD1GAlH7+LqHCUcCd2BUaiB4fwY6CoZQ=; b=JDbH87qSO3bxtPXifV+iF7rCincYzKf9kk/c3f/tLFw92owbydh/TPFnHommTyERTkWSW1 jcEFI8MbC0TDMpmU7/K2Ux3jkqwYpHSbp2Wjhu2NiY5/hZtjpnhIC0U0Bf3usWuvkyBOvj xiRhjW6g43PXBgX/T2JwDkchYg0dPFE= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-95-5b7chJ06PkOGkGkD5MbbdQ-1; Wed, 21 Oct 2020 14:49:12 -0400 X-MC-Unique: 5b7chJ06PkOGkGkD5MbbdQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 480A5876E3A; Wed, 21 Oct 2020 18:49:11 +0000 (UTC) Received: from w520.home (ovpn-112-213.phx2.redhat.com [10.3.112.213]) by smtp.corp.redhat.com (Postfix) with ESMTP id 58C9D5B4A1; Wed, 21 Oct 2020 18:49:10 +0000 (UTC) Date: Wed, 21 Oct 2020 12:49:08 -0600 From: Alex Williamson To: Paolo Bonzini Subject: Re: [PATCH 1/1] Skip flatview_simplify() for cpu vendor zhaoxin Message-ID: <20201021124908.53c7a98b@w520.home> In-Reply-To: <783b5ef0-277c-363d-f342-7c0351f9ac16@redhat.com> References: <20201016112933.14856-1-FelixCui-oc@zhaoxin.com> <20201016112933.14856-2-FelixCui-oc@zhaoxin.com> <20201019130206.1d3baffc@w520.home> <4d2b96cb-8f7f-2598-39e6-4cf0f61d567b@redhat.com> <20201020164437.5e3cb4ce@w520.home> <783b5ef0-277c-363d-f342-7c0351f9ac16@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=alex.williamson@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=alex.williamson@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/21 02:16:02 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] 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_H4=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: FelixCuioc , Eduardo Habkost , CobeChen-oc@zhaoxin.com, qemu-devel@nongnu.org, TonyWWang-oc@zhaoxin.com, RockCui-oc@zhaoxin.com, Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, 21 Oct 2020 09:37:53 +0200 Paolo Bonzini wrote: > On 21/10/20 00:44, Alex Williamson wrote: > > Do we necessarily need a memory map ioctl for this or could it be the > > QEMU code that compares the old and new maps to trigger map and unmap > > ioctls? For example (aiui) our race is that if we have contiguous > > memory regions A and B and flatview_simplify() tries to expand A and > > delete B we'll see a series of listener notifications deleting A and B > > and adding A'. But the vfio QEMU code could parse the memory map to > > determine that old A + B is functionally equivalent to A' and do > > nothing. > > I think the issue is a bit different, and in fact there are two sides of > the same issue. Say you have A (large) and it is replaced by A' > (smaller) + B, then: > > * the first part of A disappears for a moment before A' appears. This > is something that QEMU can work around, by not doing anything > > * the second part of A disappears for a moment before B appears. This > is the root API issue and not something that QEMU can work around; and > in fact it is not even fixed by removing flatview_simplify. Right, our current uAPI does not support a mechanism to atomically change a mapping, but likewise we're probably not going to have devices performing DMA to regions that are being remapped. We know that removing flatview_simplify() resolves this issue and FelixCui's update suggests we do have a case where the permission changes of an adjacent range is triggering a range consolidation, which we see as the range being removed and added as something else, larger or smaller. I can understand the general benefit of flatview_simplify(), but I wonder if the best short term solution is to skip operating on the x86 PAM range, which I understand to be a small number of memory chunks below 1MB. I might also wonder why the EHCI controller on this platform is choosing that range for DMA. Thanks, Alex