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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D0C33C433EF for ; Tue, 1 Mar 2022 22:44:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238492AbiCAWp2 (ORCPT ); Tue, 1 Mar 2022 17:45:28 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40584 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238066AbiCAWpW (ORCPT ); Tue, 1 Mar 2022 17:45:22 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 9CD5C77AB5 for ; Tue, 1 Mar 2022 14:44:40 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1646174679; 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=mGMUuYb5facBAKE4BWWQblYiu2u5yGmxdG+wUrk0CNE=; b=L+JwUzwgTLi3GUjviNXGw5wr/gwszDvyat4luhllF5qjaFn7VpZOPXu2hSyJL14+H+RCAP s9xBCZVUB00ewnLIsoQIJ9lwOeCwmX2HsQ6vDaI2pBM8t0SyFmNDZlvHWiwZlS+ApvpU9A ZIX74tB5wfBNRxnco8LfOIpoALjsSTQ= Received: from mail-ot1-f69.google.com (mail-ot1-f69.google.com [209.85.210.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-94-4LvsaXDzP5-7edusm51mkA-1; Tue, 01 Mar 2022 17:44:34 -0500 X-MC-Unique: 4LvsaXDzP5-7edusm51mkA-1 Received: by mail-ot1-f69.google.com with SMTP id f37-20020a9d2c28000000b005ad366aa68dso46473otb.8 for ; Tue, 01 Mar 2022 14:44:33 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:in-reply-to :references:organization:mime-version:content-transfer-encoding; bh=mGMUuYb5facBAKE4BWWQblYiu2u5yGmxdG+wUrk0CNE=; b=nQEsjsUlPCoLwtyPkh0GNFwYFPEyVg25th+se3HdLHZB3eyVuHhWawPEd6Rh/xwLdg CysCXIW9tQTs731ppbkNNZJXVsNHjIe+zQbVbSdg/n1JGF3SgxzLyQcowkR6yFFmY6C+ xkSTATf9wc69Zj9iNIWxEzFJYXfbSFVtN6eA7XhY+WQ2w4y/zqdpuZMID8/8fy1Fc5nl Vz9erqDpNCnOs/IJaf/JYgqHlVefr2rXMdroGmJlaff0DMh2QcikcKWBLq4M/ppy/ep9 IETmFABCvmfzZ5jVjaBEZAR7ypGrDai3EDRCe8n0bLQ0HXTKwf6iNcrS2kkM3ajPtRML 0tgQ== X-Gm-Message-State: AOAM532nfCtQ5piSBmOwROjlPuscFgDULocpBJ/4oK3itjAk/0Wf1N71 3Zuky0AyyI5fEPA2t6KyhY1kB5GClRBoZXIQQRs6iTcV3jLeGLEiqVLOU6CaWkfDctO5/mELOSj ivdnr4tQDnn73 X-Received: by 2002:a05:6870:65a0:b0:d7:547:3c0b with SMTP id fp32-20020a05687065a000b000d705473c0bmr10477958oab.79.1646174673349; Tue, 01 Mar 2022 14:44:33 -0800 (PST) X-Google-Smtp-Source: ABdhPJw0+uzXIFvxXkkhsLvg5Psp36LJMpZFA1kLIP23iFYOblelNGSikei+BKT4/FUoBnmsURZE9Q== X-Received: by 2002:a05:6870:65a0:b0:d7:547:3c0b with SMTP id fp32-20020a05687065a000b000d705473c0bmr10477943oab.79.1646174673084; Tue, 01 Mar 2022 14:44:33 -0800 (PST) Received: from redhat.com ([38.15.36.239]) by smtp.gmail.com with ESMTPSA id f10-20020a9d5e8a000000b0059fa2fa9b4bsm6918556otl.13.2022.03.01.14.44.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Mar 2022 14:44:32 -0800 (PST) Date: Tue, 1 Mar 2022 15:44:31 -0700 From: Alex Williamson To: Jason Gunthorpe Cc: Shameerali Kolothum Thodi , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-crypto@vger.kernel.org" , "cohuck@redhat.com" , "mgurtovoy@nvidia.com" , "yishaih@nvidia.com" , Linuxarm , liulongfang , "Zengtao (B)" , Jonathan Cameron , "Wangzhou (B)" Subject: Re: [PATCH v6 09/10] hisi_acc_vfio_pci: Add support for VFIO live migration Message-ID: <20220301154431.42b27278.alex.williamson@redhat.com> In-Reply-To: <20220301203938.GY219866@nvidia.com> References: <20220228145731.GH219866@nvidia.com> <58fa5572e8e44c91a77bd293b2ec6e33@huawei.com> <20220228180520.GO219866@nvidia.com> <20220228131614.27ad37dc.alex.williamson@redhat.com> <20220228202919.GP219866@nvidia.com> <20220228142034.024e7be6.alex.williamson@redhat.com> <20220228234709.GV219866@nvidia.com> <20220228214110.4deb551f.alex.williamson@redhat.com> <20220301131528.GW219866@nvidia.com> <20220301123047.1171c730.alex.williamson@redhat.com> <20220301203938.GY219866@nvidia.com> Organization: Red Hat MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, 1 Mar 2022 16:39:38 -0400 Jason Gunthorpe wrote: > On Tue, Mar 01, 2022 at 12:30:47PM -0700, Alex Williamson wrote: > > Wouldn't it make more sense if initial-bytes started at QM_MATCH_SIZE > > and dirty-bytes was always sizeof(vf_data) - QM_MATCH_SIZE? ie. QEMU > > would know that it has sizeof(vf_data) - QM_MATCH_SIZE remaining even > > while it's getting ENOMSG after reading QM_MATCH_SIZE bytes of data. > > The purpose of this ioctl is to help userspace guess when moving on to > STOP_COPY is a good idea ie when the device has done almost all the > work it is going to be able to do in PRE_COPY. ENOMSG is a similar > indicator. > > I expect all devices to have some additional STOP_COPY trailer_data in > addition to their PRE_COPY initial_data and dirty_data > > There is a choice to make if we report the trailer_data during > PRE_COPY or not. As this is all estimates, it doesn't matter unless > the trailer_data is very big. > > Having all devices trend toward a 0 dirty_bytes to say they are are > done all the pre-copy they can do makes sense from an API > perspective. If one device trends toward 10MB due to a big > trailer_data and one trends toward 0 bytes, how will qemu consistently > decide when best to trigger STOP_COPY? It makes the API less useful. > > So, I would not include trailer_data in the dirty_bytes. That assumes that it's possible to keep up with the device dirty rate. It seems like a better approach for userspace would be to look at how dirty_bytes is trending. A zero value and a steady state value are equivalent, there's nothing more to be gained by further iterations. If the value is trending down, it might be worthwhile to iterate in PRE_COPY a while longer. If the value is trending up, it might be time to cut to STOP_COPY or abort the migration. If we exclude STOP_COPY trailing data from the VFIO_DEVICE_MIG_PRECOPY ioctl, it seems even more of a disconnect that when we enter the STOP_COPY state, suddenly we start getting new data out of a PRECOPY ioctl. BTW, "VFIO_DEVICE" should be reserved for ioctls and data structures relative to the device FD, appending it with _MIG is too subtle for me. This is also a GET operation for INFO, so I'd think for consistency with the existing vfio uAPI we'd name this something like VFIO_MIG_GET_PRECOPY_INFO where the structure might be named vfio_precopy_info. > Estimating when to move on to STOP_COPY and trying to enforce a SLA on > STOP_COPY are different tasks and will probably end up with different > interfaces. > > I still think the right way to approach the SLA is to inform the > driver what the permitted time and data size target is for STOP_COPY > and the driver can proceed or not based on its own internal > calculation. So if we don't think this is the right approach for STOP_COPY, then why are we pushing that it has any purpose outside of PRECOPY or might be implemented by a non-PRECOPY driver for use in STOP_COPY? > > useful yet and you don't want to add dead kernel code, then let's > > define that this ioctl is only available in the PRE_COPY* states and > > returns -errno in the STOP_COPY state. > > I'm OK with that, in acc it is done by checking migf->total_bytes > > QM_MATCH_SIZE during the read fop > > > devices in STOP_COPY and let's also define if there's actually anything > > userspace can infer about remaining STOP_COPY data size while in > > PRE_COPY* via this ioctl. For example, is dirty-bytes zero or the > > remaining data structure size? > > If we keep it then I would say it doesn't matter, userspace has to sum > the two values to get the total remaining length estimate, it is just > a bit quirky. For the reasons above, I just can't figure out why wouldn't decide that use of this outside of PRECOPY is too quirky to bother with. Thanks, Alex