From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Sender: 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 A2FA49865D8 for ; Fri, 3 Sep 2021 09:38:53 +0000 (UTC) References: <87v94ldrqq.fsf@linaro.org> <875yvkh1p1.fsf@linaro.org> <20210903080609.GD47953@laputa> From: Alex =?utf-8?Q?Benn=C3=A9e?= Date: Fri, 03 Sep 2021 10:28:06 +0100 In-reply-to: <20210903080609.GD47953@laputa> Message-ID: <87czpqq9qu.fsf@linaro.org> MIME-Version: 1.0 Subject: [virtio-dev] Re: Enabling hypervisor agnosticism for VirtIO backends Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable To: AKASHI Takahiro Cc: Stefan Hajnoczi , Stefano Stabellini , Stratos Mailing List , virtio-dev@lists.oasis-open.org, Arnd Bergmann , Viresh Kumar , Stefano Stabellini , Jan Kiszka , Carl van Schaik , pratikp@quicinc.com, Srivatsa Vaddagiri , Jean-Philippe Brucker , Mathieu Poirier , Wei.Chen@arm.com, olekstysh@gmail.com, Oleksandr_Tyshchenko@epam.com, Bertrand.Marquis@arm.com, Artem_Mygaiev@epam.com, julien@xen.org, jgross@suse.com, paul@xen.org, xen-devel@lists.xen.org, Elena Afanasova List-ID: AKASHI Takahiro writes: > Alex, > > On Wed, Sep 01, 2021 at 01:53:34PM +0100, Alex Benn??e wrote: >>=20 >> Stefan Hajnoczi writes: >>=20 >> > [[PGP Signed Part:Undecided]] >> > On Wed, Aug 04, 2021 at 12:20:01PM -0700, Stefano Stabellini wrote: >> >> > Could we consider the kernel internally converting IOREQ messages f= rom >> >> > the Xen hypervisor to eventfd events? Would this scale with other k= ernel >> >> > hypercall interfaces? >> >> >=20 >> >> > So any thoughts on what directions are worth experimenting with? >> >> =20 >> >> One option we should consider is for each backend to connect to Xen v= ia >> >> the IOREQ interface. We could generalize the IOREQ interface and make= it >> >> hypervisor agnostic. The interface is really trivial and easy to add. >> >> The only Xen-specific part is the notification mechanism, which is an >> >> event channel. If we replaced the event channel with something else t= he >> >> interface would be generic. See: >> >> https://gitlab.com/xen-project/xen/-/blob/staging/xen/include/public/= hvm/ioreq.h#L52 >> > >> > There have been experiments with something kind of similar in KVM >> > recently (see struct ioregionfd_cmd): >> > https://lore.kernel.org/kvm/dad3d025bcf15ece11d9df0ff685e8ab0a4f2edd.1= 613828727.git.eafanasova@gmail.com/ >>=20 >> Reading the cover letter was very useful in showing how this provides a >> separate channel for signalling IO events to userspace instead of using >> the normal type-2 vmexit type event. I wonder how deeply tied the >> userspace facing side of this is to KVM? Could it provide a common FD >> type interface to IOREQ? > > Why do you stick to a "FD" type interface? I mean most user space interfaces on POSIX start with a file descriptor and the usual read/write semantics or a series of ioctls. >> As I understand IOREQ this is currently a direct communication between >> userspace and the hypervisor using the existing Xen message bus. My > > With IOREQ server, IO event occurrences are notified to BE via Xen's even= t > channel, while the actual contexts of IO events (see struct ioreq in iore= q.h) > are put in a queue on a single shared memory page which is to be assigned > beforehand with xenforeignmemory_map_resource hypervisor call. If we abstracted the IOREQ via the kernel interface you would probably just want to put the ioreq structure on a queue rather than expose the shared page to userspace.=20 >> worry would be that by adding knowledge of what the underlying >> hypervisor is we'd end up with excess complexity in the kernel. For one >> thing we certainly wouldn't want an API version dependency on the kernel >> to understand which version of the Xen hypervisor it was running on. > > That's exactly what virtio-proxy in my proposal[1] does; All the hypervis= or- > specific details of IO event handlings are contained in virtio-proxy > and virtio BE will communicate with virtio-proxy through a virtqueue > (yes, virtio-proxy is seen as yet another virtio device on BE) and will > get IO event-related *RPC* callbacks, either MMIO read or write, from > virtio-proxy. > > See page 8 (protocol flow) and 10 (interfaces) in [1]. There are two areas of concern with the proxy approach at the moment. The first is how the bootstrap of the virtio-proxy channel happens and the second is how many context switches are involved in a transaction. Of course with all things there is a trade off. Things involving the very tightest latency would probably opt for a bare metal backend which I think would imply hypervisor knowledge in the backend binary. > > If kvm's ioregionfd can fit into this protocol, virtio-proxy for kvm > will hopefully be implemented using ioregionfd. > > -Takahiro Akashi > > [1] https://op-lists.linaro.org/pipermail/stratos-dev/2021-August/000548.= html --=20 Alex Benn=C3=A9e --------------------------------------------------------------------- To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org