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=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 C07BDC282DD for ; Thu, 9 Jan 2020 16:28:45 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 8B3C92067D for ; Thu, 9 Jan 2020 16:28:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="B5ykeiCN" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8B3C92067D 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 ([::1]:34924 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ipafs-0005Mu-Oa for qemu-devel@archiver.kernel.org; Thu, 09 Jan 2020 11:28:44 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:50784) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1ipafB-0004l9-0s for qemu-devel@nongnu.org; Thu, 09 Jan 2020 11:28:02 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ipaf7-00086j-Ih for qemu-devel@nongnu.org; Thu, 09 Jan 2020 11:27:58 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:22368 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ipaf7-0007zM-04 for qemu-devel@nongnu.org; Thu, 09 Jan 2020 11:27:57 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1578587276; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=FpuGEH+efGC2/lc8mf2D/omKm/DYUYu5LDMiAooiHFc=; b=B5ykeiCNqLrST+2lGrRQ5JZxk0/B41lVgl2IXkpYPuGKFZB+ybEQR91T91H6rITiSqL3H5 xtNMSSXkii91aXljBuZL5tsqxNdfsRDUK40BmRrcSQqbIfiIXujAP6VQ9/JYusY00uiG3f wCdXcXn9s0vt48ADXO0fjJM3yHUxmLg= 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-274-KPEF5CQXOXCAFXC7gEn2vg-1; Thu, 09 Jan 2020 11:27:52 -0500 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 mimecast-mx01.redhat.com (Postfix) with ESMTPS id B7B3E1194C71; Thu, 9 Jan 2020 16:27:51 +0000 (UTC) Received: from work-vm (unknown [10.36.118.29]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1BD885C290; Thu, 9 Jan 2020 16:27:47 +0000 (UTC) Date: Thu, 9 Jan 2020 16:27:45 +0000 From: "Dr. David Alan Gilbert" To: Roman Kagan , Vitaly Kuznetsov , qemu-devel@nongnu.org, jasowang@redhat.com, "Michael S. Tsirkin" Subject: Re: [PATCH 2/2] hyperv/synic: Allocate as ram_device Message-ID: <20200109162745.GL6795@work-vm> References: <20200108135353.75471-1-dgilbert@redhat.com> <20200108135353.75471-3-dgilbert@redhat.com> <20200109064527-mutt-send-email-mst@kernel.org> <20200109120820.GB6795@work-vm> <20200109071454-mutt-send-email-mst@kernel.org> <20200109122237.GD6795@work-vm> <87r208rdin.fsf@vitty.brq.redhat.com> <20200109132432.GD3147@rkaganb.sw.ru> <20200109132821.GG6795@work-vm> <20200109161156.GE3147@rkaganb.sw.ru> MIME-Version: 1.0 In-Reply-To: <20200109161156.GE3147@rkaganb.sw.ru> User-Agent: Mutt/1.13.0 (2019-11-30) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-MC-Unique: KPEF5CQXOXCAFXC7gEn2vg-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.120 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Roman Kagan (rkagan@virtuozzo.com) wrote: > On Thu, Jan 09, 2020 at 01:28:21PM +0000, Dr. David Alan Gilbert wrote: > > * Roman Kagan (rkagan@virtuozzo.com) wrote: > > > On Thu, Jan 09, 2020 at 02:00:00PM +0100, Vitaly Kuznetsov wrote: > > > > "Dr. David Alan Gilbert" writes: > > > >=20 > > > > > And I think vhost-user will fail if you have too many sections - = and > > > > > the 16 sections from synic I think will blow the slots available. > > > > > > > > >=20 > > > > SynIC is percpu, it will allocate two 4k pages for every vCPU the g= uest > > > > has so we're potentially looking at hundreds of such regions. > > >=20 > > > Indeed. > > >=20 > > > I think my original idea to implement overlay pages word-for-word to = the > > > HyperV spec was a mistake, as it lead to fragmentation and memslot > > > waste. > > >=20 > > > I'll look into reworking it without actually mapping extra pages over > > > the existing RAM, but achieving overlay semantics by just shoving the > > > *content* of the "overlaid" memory somewhere. > > >=20 > > > That said, I haven't yet fully understood how the reported issue came > > > about, and thus whether the proposed approach would resolve it too. > >=20 > > The problem happens when we end up with: > >=20 > > a) 0-512k RAM > > b) 512k + synic > > c) 570kish-640k RAM > >=20 > > the page alignment code rounds > > (a) to 0-2MB - aligning to the hugepage it's in > > (b) leaves as is > > (c) aligns to 0-2MB > >=20 > > it then tries to coalesce (c) and (a) and notices (b) got in the way > > and fails it. >=20 > I see, thanks. The only bit I still haven't quite followed is how this > failure results in a quiet vhost malfunction rather than a refusal to > start vhost. Because there's no way to fail in vhost_region_add_section other than to abort; if (mrs_gpa < prev_gpa_start) { error_report("%s:Section rounded to %"PRIx64 " prior to previous %"PRIx64, __func__, mrs_gpa, prev_gpa_start); /* A way to cleanly fail here would be better */ return; } > > Given the guest can put Synic anywhere I'm not sure that changing it's > > implementatino would help here. >=20 > There would be no (b) nor (separate) (c): synic would just refer to some > memory straight from (a), regardless of its paging granularity. Oh, if it's actually memory from main RAM, then sure, but I guess you'd have to reserve that somehow to stop the OS using it. > > (And changing it's implementation would probably break migration > > compatibility). >=20 > I'm afraid I see no better option. Migration compatibility! Dave > Thanks, > Roman. >=20 -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK