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 133F7C433F5 for ; Wed, 2 Mar 2022 16:57:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S243567AbiCBQ6h (ORCPT ); Wed, 2 Mar 2022 11:58:37 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44942 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243581AbiCBQ6c (ORCPT ); Wed, 2 Mar 2022 11:58:32 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 0B2D021E16 for ; Wed, 2 Mar 2022 08:57:48 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1646240268; 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: in-reply-to:in-reply-to:references:references; bh=tmS5o/o7TOxgF/ChRH9DIbCR+xpKRUvssVTS2w4RtUs=; b=COx6V6YMCVLkJKZwhT/8xUFcaKoMsysd7Y6pj0QX6puiOsTMFcHh49CMy6+FeIABk3H9oM 0kzhBIJVASVfPNBsHdZ5ZE6GFVSPBxOm/J+fSm+232G3iR1fG//GSJPwqfaEelXIZL1rSp XjXwrD9wi0ci2FNVGnCiza0PlgTsu08= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-37-dgeZSHUkP-W5PjRyWKQeow-1; Wed, 02 Mar 2022 11:57:41 -0500 X-MC-Unique: dgeZSHUkP-W5PjRyWKQeow-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id F3424824FA6; Wed, 2 Mar 2022 16:57:38 +0000 (UTC) Received: from localhost (unknown [10.39.194.94]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 246F72DE6B; Wed, 2 Mar 2022 16:57:26 +0000 (UTC) From: Cornelia Huck To: Yishai Hadas , alex.williamson@redhat.com, bhelgaas@google.com, jgg@nvidia.com, saeedm@nvidia.com Cc: linux-pci@vger.kernel.org, kvm@vger.kernel.org, netdev@vger.kernel.org, kuba@kernel.org, leonro@nvidia.com, kwankhede@nvidia.com, mgurtovoy@nvidia.com, yishaih@nvidia.com, maorg@nvidia.com, ashok.raj@intel.com, kevin.tian@intel.com, shameerali.kolothum.thodi@huawei.com Subject: Re: [PATCH V9 mlx5-next 09/15] vfio: Define device migration protocol v2 In-Reply-To: <20220224142024.147653-10-yishaih@nvidia.com> Organization: Red Hat GmbH References: <20220224142024.147653-1-yishaih@nvidia.com> <20220224142024.147653-10-yishaih@nvidia.com> User-Agent: Notmuch/0.34 (https://notmuchmail.org) Date: Wed, 02 Mar 2022 17:57:25 +0100 Message-ID: <87h78gi96y.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Thu, Feb 24 2022, Yishai Hadas wrote: > From: Jason Gunthorpe > > Replace the existing region based migration protocol with an ioctl based > protocol. The two protocols have the same general semantic behaviors, but > the way the data is transported is changed. > > This is the STOP_COPY portion of the new protocol, it defines the 5 states > for basic stop and copy migration and the protocol to move the migration > data in/out of the kernel. > > Compared to the clarification of the v1 protocol Alex proposed: > > https://lore.kernel.org/r/163909282574.728533.7460416142511440919.stgit@omen > > This has a few deliberate functional differences: > > - ERROR arcs allow the device function to remain unchanged. > > - The protocol is not required to return to the original state on > transition failure. Instead userspace can execute an unwind back to > the original state, reset, or do something else without needing kernel > support. This simplifies the kernel design and should userspace choose > a policy like always reset, avoids doing useless work in the kernel > on error handling paths. > > - PRE_COPY is made optional, userspace must discover it before using it. > This reflects the fact that the majority of drivers we are aware of > right now will not implement PRE_COPY. > > - segmentation is not part of the data stream protocol, the receiver > does not have to reproduce the framing boundaries. > > The hybrid FSM for the device_state is described as a Mealy machine by > documenting each of the arcs the driver is required to implement. Defining > the remaining set of old/new device_state transitions as 'combination > transitions' which are naturally defined as taking multiple FSM arcs along > the shortest path within the FSM's digraph allows a complete matrix of > transitions. > > A new VFIO_DEVICE_FEATURE of VFIO_DEVICE_FEATURE_MIG_DEVICE_STATE is > defined to replace writing to the device_state field in the region. This > allows returning a brand new FD whenever the requested transition opens > a data transfer session. > > The VFIO core code implements the new feature and provides a helper > function to the driver. Using the helper the driver only has to > implement 6 of the FSM arcs and the other combination transitions are > elaborated consistently from those arcs. > > A new VFIO_DEVICE_FEATURE of VFIO_DEVICE_FEATURE_MIGRATION is defined to > report the capability for migration and indicate which set of states and > arcs are supported by the device. The FSM provides a lot of flexibility to > make backwards compatible extensions but the VFIO_DEVICE_FEATURE also > allows for future breaking extensions for scenarios that cannot support > even the basic STOP_COPY requirements. > > The VFIO_DEVICE_FEATURE_MIG_DEVICE_STATE with the GET option (i.e. > VFIO_DEVICE_FEATURE_GET) can be used to read the current migration state > of the VFIO device. > > Data transfer sessions are now carried over a file descriptor, instead of > the region. The FD functions for the lifetime of the data transfer > session. read() and write() transfer the data with normal Linux stream FD > semantics. This design allows future expansion to support poll(), > io_uring, and other performance optimizations. > > The complicated mmap mode for data transfer is discarded as current qemu > doesn't take meaningful advantage of it, and the new qemu implementation > avoids substantially all the performance penalty of using a read() on the > region. > > Signed-off-by: Jason Gunthorpe > Tested-by: Shameer Kolothum > Reviewed-by: Kevin Tian > Signed-off-by: Yishai Hadas > --- > drivers/vfio/vfio.c | 199 ++++++++++++++++++++++++++++++++++++++ > include/linux/vfio.h | 20 ++++ > include/uapi/linux/vfio.h | 174 ++++++++++++++++++++++++++++++--- > 3 files changed, 380 insertions(+), 13 deletions(-) Reviewed-by: Cornelia Huck