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Subject: Re: [RFC PATCH v2] docs/interop: define PROBE feature for vhost-user VirtIO devices
On Fri, Sep 08, 2023 at 01:03:26PM +0100, Alex Bennée wrote: > > Stefan Hajnoczi <stefanha@redhat.com> writes: > > > On Fri, Sep 01, 2023 at 12:00:18PM +0100, Alex Bennée 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. > >> > >> 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. > >> > >> 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. > >> > >> Signed-off-by: Alex Bennée <alex.bennee@linaro.org> > > > > 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. > > 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. > > 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: > > #define CCW_CMD_SET_VQ 0x13 > #define CCW_CMD_VDEV_RESET 0x33 > #define CCW_CMD_SET_IND 0x43 > #define CCW_CMD_SET_CONF_IND 0x53 > #define CCW_CMD_SET_IND_ADAPTER 0x73 > #define CCW_CMD_READ_FEAT 0x12 > #define CCW_CMD_WRITE_FEAT 0x11 > #define CCW_CMD_READ_CONF 0x22 > #define CCW_CMD_WRITE_CONF 0x21 > #define CCW_CMD_WRITE_STATUS 0x31 > #define CCW_CMD_READ_VQ_CONF 0x32 > #define CCW_CMD_SET_VIRTIO_REV 0x83 > #define CCW_CMD_READ_STATUS 0x72 > > 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. > > I agree we want to do this right. > > > 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. > > 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. Stefan
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