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 ws5-mx01.kavi.com (ws5-mx01.kavi.com [34.193.7.191]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 03D7EEE7FEF for ; Fri, 8 Sep 2023 13:01:17 +0000 (UTC) Received: from lists.oasis-open.org (oasis.ws5.connectedcommunity.org [10.110.1.242]) by ws5-mx01.kavi.com (Postfix) with ESMTP id 4649DEEA14 for ; Fri, 8 Sep 2023 13:01:17 +0000 (UTC) Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id 150E1986605 for ; Fri, 8 Sep 2023 13:01:17 +0000 (UTC) Received: from host09.ws5.connectedcommunity.org (host09.ws5.connectedcommunity.org [10.110.1.97]) by lists.oasis-open.org (Postfix) with QMQP id EE0C598644B; Fri, 8 Sep 2023 13:01:16 +0000 (UTC) Mailing-List: contact virtio-dev-help@lists.oasis-open.org; run by ezmlm List-ID: Sender: Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: Received: from lists.oasis-open.org (oasis-open.org [10.110.1.242]) by lists.oasis-open.org (Postfix) with ESMTP id DBE8B98644C for ; Fri, 8 Sep 2023 13:01:16 +0000 (UTC) X-Virus-Scanned: amavisd-new at kavi.com X-MC-Unique: XHa_rEn6M5y8aqpl4SPiSw-1 Date: Fri, 8 Sep 2023 09:01:08 -0400 From: Stefan Hajnoczi To: Alex =?iso-8859-1?Q?Benn=E9e?= Cc: slp@redhat.com, mst@redhat.com, marcandre.lureau@redhat.com, viresh.kumar@linaro.org, sgarzare@redhat.com, takahiro.akashi@linaro.org, erik.schilling@linaro.org, manos.pitsidianakis@linaro.org, mathieu.poirier@linaro.org, qemu-devel@nongnu.org, virtio-dev@lists.oasis-open.org, virtio-comment@lists.oasis-open.org Message-ID: <20230908130108.GA3561353@fedora> References: <20230901110018.3704459-1-alex.bennee@linaro.org> <20230907192259.GC1560640@fedora> <87v8ckzx0g.fsf@linaro.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="4lYbCNRBzkY5d5b4" Content-Disposition: inline In-Reply-To: <87v8ckzx0g.fsf@linaro.org> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Subject: [virtio-dev] Re: [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices --4lYbCNRBzkY5d5b4 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Fri, Sep 08, 2023 at 01:03:26PM +0100, Alex Benn=E9e wrote: >=20 > Stefan Hajnoczi writes: >=20 > > On Fri, Sep 01, 2023 at 12:00:18PM +0100, Alex Benn=E9e wrote: > >> Currently QEMU has to know some details about the VirtIO device > >> supported by a vhost-user daemon to be able to setup the guest. This > >> makes it hard for QEMU to add support for additional vhost-user > >> daemons without adding specific stubs for each additional VirtIO > >> device. > >>=20 > >> This patch suggests a new feature flag (VHOST_USER_PROTOCOL_F_PROBE) > >> which the back-end can advertise which allows a probe message to be > >> sent to get all the details QEMU needs to know in one message. > >>=20 > >> Together with the existing features VHOST_USER_PROTOCOL_F_STATUS and > >> VHOST_USER_PROTOCOL_F_CONFIG we can create "standalone" vhost-user > >> daemons which are capable of handling all aspects of the VirtIO > >> transactions with only a generic stub on the QEMU side. These daemons > >> can also be used without QEMU in situations where there isn't a full > >> VMM managing their setup. > >>=20 > >> Signed-off-by: Alex Benn=E9e > > > > I think the mindset for this change should be "vhost-user is becoming a > > VIRTIO Transport". VIRTIO Transports have a reasonably well-defined > > feature set in the VIRTIO specification. The goal should be to cover > > every VIRTIO Transport operation via vhost-user protocol messages so > > that the VIRTIO device model can be fully conveyed over vhost-user. >=20 > Is it though? The transport is a guest visible construct whereas > vhost-user is purely a backend implementation detail that should be > invisible to the guest. No, the transport is not necessarily guest-visible. The vhost-user model is that the front-end emulates a VIRTIO device and some aspects of that device are delegated to the vhost-user back-end. In other words, the vhost-user device is not the same as the VIRTIO device that the guest sees, but it's still important for the vhost-user back-end to be a VIRTIO Transport because that's how we can be sure it supports the VIRTIO device model properly. >=20 > Also the various backends do things a different set of ways. The > differences between MMIO and PCI are mostly around where config space is > and how IRQs are handled. For CCW we do actually have a set of commands > we can look at: >=20 > #define CCW_CMD_SET_VQ 0x13=20 > #define CCW_CMD_VDEV_RESET 0x33=20 > #define CCW_CMD_SET_IND 0x43=20 > #define CCW_CMD_SET_CONF_IND 0x53=20 > #define CCW_CMD_SET_IND_ADAPTER 0x73=20 > #define CCW_CMD_READ_FEAT 0x12=20 > #define CCW_CMD_WRITE_FEAT 0x11=20 > #define CCW_CMD_READ_CONF 0x22=20 > #define CCW_CMD_WRITE_CONF 0x21=20 > #define CCW_CMD_WRITE_STATUS 0x31=20 > #define CCW_CMD_READ_VQ_CONF 0x32=20 > #define CCW_CMD_SET_VIRTIO_REV 0x83=20 > #define CCW_CMD_READ_STATUS 0x72 >=20 > which I think we already have mappings for. Yes, there are differences between the transports. vhost-user uses eventfds (callfd/kickfd) instead of interrupts. > > Anything less is yet another ad-hoc protocol extension that will lead to > > more bugs and hacks when it turns out some VIRTIO devices cannot be > > expressed due to limitations in the protocol. >=20 > I agree we want to do this right. >=20 > > This requires going through the VIRTIO spec to find a correspondence > > between virtio-pci/virtio-mmio/virtio-ccw's interfaces and vhost-user > > protocol messages. In most cases vhost-user already offers messages and > > your patch adds more of what is missing. I think this effort is already > > very close but missing the final check that it really matches the VIRTIO > > spec. > > > > Please do the comparison against the VIRTIO Transports and then adjust > > this patch to make it clear that the back-end is becoming a full-fledged > > VIRTIO Transport: > > - The name of the patch series should reflect that. > > - The vhost-user protocol feature should be named F_TRANSPORT. > > - The messages added in this patch should have a 1:1 correspondence with > > the VIRTIO spec including using the same terminology for consistency. > > > > Sorry for the hassle, but I think this is a really crucial point where > > we have the chance to make vhost-user work smoothly in the future...but > > only if we can faithfully expose VIRTIO Transport semantics. >=20 > I wonder if first be handled by cleaning up the VirtIO spec to make it > clear what capabilities each transport needs to support? It's a fair point that the VIRTIO spec does not provide an interface definition for the VIRTIO Transport or at least a definitive list of requirements. The requirements are implicit (i.e. it is assumed that very transport provides a way to set the virtqueue descriptor table addresses) so it's necessary to review the existing transports to understand their functionality. If you want to create a list of the requirements for a VIRTIO Transport and propose a patch to the VIRTIO spec then that would be great, but I don't think that stops this patch series. It's possible to review virtio-pci/virtio-mmio/virtio-ccw and check that there is equivalent functionality in the vhost-user protocol. 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