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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 030F2C5DF61 for ; Thu, 7 Nov 2019 11:17:44 +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 BF6CC2187F for ; Thu, 7 Nov 2019 11:17:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="S2nFFohY" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BF6CC2187F 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]:41004 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iSfnK-00068x-VZ for qemu-devel@archiver.kernel.org; Thu, 07 Nov 2019 06:17:42 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:47201) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iSfmG-0005WE-Ee for qemu-devel@nongnu.org; Thu, 07 Nov 2019 06:16:37 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iSfmE-0001Cw-HT for qemu-devel@nongnu.org; Thu, 07 Nov 2019 06:16:35 -0500 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:43040 helo=us-smtp-delivery-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iSfmE-0001CY-Dd for qemu-devel@nongnu.org; Thu, 07 Nov 2019 06:16:34 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1573125393; 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=xAjtWTZFGVGmU+z+TlpgDIFQ7RxMzAz9ig4BilDW1QY=; b=S2nFFohYpbm1D/8MXQYRR/8wWIA004fuILqkjlXSf9PB+yVhtHrM40WhuyPUTU4TIKZq31 H8bCRWYeZK7XAKW2y5P/J/pgQ/N6FT/ARcVJbGDSNu1toXI2sHhve6EKz49Y9hfDTyxkRm ZIiAKsmIAMLO/UjONMaa+8a75y1eYDA= 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-397-XQ_TWMOzOcuypzi0dH6vLA-1; Thu, 07 Nov 2019 06:16:30 -0500 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 321D51800D7A; Thu, 7 Nov 2019 11:16:28 +0000 (UTC) Received: from work-vm (unknown [10.36.118.14]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8F837600F0; Thu, 7 Nov 2019 11:16:21 +0000 (UTC) Date: Thu, 7 Nov 2019 11:16:18 +0000 From: "Dr. David Alan Gilbert" To: Gerd Hoffmann Subject: Re: [virtio-dev] Re: guest / host buffer sharing ... Message-ID: <20191107111618.GE2816@work-vm> References: <20191105105456.7xbhtistnbp272lj@sirius.home.kraxel.org> <20191106084344.GB189998@stefanha-x1.localdomain> <20191106095122.jju7eo57scfoat6a@sirius.home.kraxel.org> <20191106101057.GC2802@work-vm> <20191107111119.qgr2qxgdf64jurin@sirius.home.kraxel.org> MIME-Version: 1.0 In-Reply-To: <20191107111119.qgr2qxgdf64jurin@sirius.home.kraxel.org> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-MC-Unique: XQ_TWMOzOcuypzi0dH6vLA-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=WINDOWS-1252 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: 207.211.31.81 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: geoff@hostfission.com, virtio-dev@lists.oasis-open.org, Alex Lau , Alexandre Courbot , Stefan Hajnoczi , qemu-devel@nongnu.org, Tomasz Figa , Keiichi Watanabe , David Stevens , Daniel Vetter , =?iso-8859-1?Q?St=E9phane?= Marchesin , Dylan Reid , Gurchetan Singh , Hans Verkuil , Dmitry Morozov , Pawel Osciak , Linux Media Mailing List Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Gerd Hoffmann (kraxel@redhat.com) wrote: > Hi, >=20 > > > This is not about host memory, buffers are in guest ram, everything e= lse > > > would make sharing those buffers between drivers inside the guest (as > > > dma-buf) quite difficult. > >=20 > > Given it's just guest memory, can the guest just have a virt queue on > > which it places pointers to the memory it wants to share as elements in > > the queue? >=20 > Well, good question. I'm actually wondering what the best approach is > to handle long-living, large buffers in virtio ... >=20 > virtio-blk (and others) are using the approach you describe. They put a > pointer to the io request header, followed by pointer(s) to the io > buffers directly into the virtqueue. That works great with storage for > example. The queue entries are tagged being "in" or "out" (driver to > device or visa-versa), so the virtio transport can set up dma mappings > accordingly or even transparently copy data if needed. >=20 > For long-living buffers where data can potentially flow both ways this > model doesn't fit very well though. So what virtio-gpu does instead is > transferring the scatter list as virtio payload. Does feel a bit > unclean as it doesn't really fit the virtio architecture. It assumes > the host can directly access guest memory for example (which is usually > the case but explicitly not required by virtio). It also requires > quirks in virtio-gpu to handle VIRTIO_F_IOMMU_PLATFORM properly, which > in theory should be handled fully transparently by the virtio-pci > transport. >=20 > We could instead have a "create-buffer" command which adds the buffer > pointers as elements to the virtqueue as you describe. Then simply > continue using the buffer even after completing the "create-buffer" > command. Which isn't exactly clean either. It would likewise assume > direct access to guest memory, and it would likewise need quirks for > VIRTIO_F_IOMMU_PLATFORM as the virtio-pci transport tears down the dma > mappings for the virtqueue entries after command completion. >=20 > Comments, suggestions, ideas? What about not completing the command while the device is using the memory? Dave > cheers, > Gerd >=20 -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK