From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: virtio-comment-return-594-cohuck=redhat.com@lists.oasis-open.org 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 4E494985E9F for ; Wed, 23 Jan 2019 15:12:23 +0000 (UTC) Date: Wed, 23 Jan 2019 10:12:14 -0500 From: "Michael S. Tsirkin" Message-ID: <20190123100932-mutt-send-email-mst@kernel.org> References: <20190111114200.10026-1-dgilbert@redhat.com> <20190111114200.10026-2-dgilbert@redhat.com> <20190111131540.12a8abca.cohuck@redhat.com> <20190111122654.GE2738@work-vm> <20190115111038.6769d292.cohuck@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190115111038.6769d292.cohuck@redhat.com> Subject: Re: [virtio-comment] [PATCH 1/3] shared memory: Define shared memory regions To: Cornelia Huck Cc: "Dr. David Alan Gilbert" , virtio-comment@lists.oasis-open.org, stefanha@redhat.com, Halil Pasic List-ID: On Tue, Jan 15, 2019 at 11:10:38AM +0100, Cornelia Huck wrote: > On Fri, 11 Jan 2019 12:26:54 +0000 > "Dr. David Alan Gilbert" wrote: > > > * Cornelia Huck (cohuck@redhat.com) wrote: > > > On Fri, 11 Jan 2019 11:41:58 +0000 > > > "Dr. David Alan Gilbert (git)" wrote: > > > > > +\section{Shared Memory Regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions} > > > > + > > > > +Shared memory regions are an additional facility > > > > +available to devices that need a region of memory that's > > > > +continuously shared between the host and the guest, rather > > > > +than passed between them in the way virtqueue elements are. > > I think we probably need to clarify the expectations (consistency etc.) > a bit more, see my remarks below. > > > > > + > > > > +Example uses include shared caches and version pools for versioned > > > > +data structures. > > > > + > > > > +Shared memory regions MUST NOT be used to control the operation > > > > +of the device, nor to stream data; those should still be performed > > > > +using virtqueues. > > > > + > > > > +A device may have multiple shared memory regions associated with > > > > +it. Each region has a \field{shmid} to identify it, the meaning > > > > +of which is device specific. > > > > + > > > > +Enumeration and location of shared memory regions is performed > > > > +using a transport-specific data structure. > > > > > > "data structure and mechanism"? > > > > Changed; thanks. > > > > > > + > > > > +The guest physical address and the host virtual address MUST NOT > > > > +be used to identify structures within the memory regions; all > > > > +addressing MUST be relative to the start of a particular region. > > > > + > > > > > > Is the intended implementation that the device provides a certain > > > memory region (in host memory) and exposes it to the driver? Are there > > > supposed to be any notifications of writes? Or do both simply write to > > > the region and get whatever updates the other side has made when they > > > read from the region again? > > > > There's no notification; in our case we have two main uses: > > a) Direct mapping of host files into the guests memory > > > > b) Mapping of a version table with quickly updated version numbers for > > data structures to do quick invalidation > > This sounds a lot like "we have a memory area, and both device and > driver may write to or read from it at any time". Are there any > expectations regarding consistency when reading data, or is there > supposed to be a device-type specific mechanism to get certain > consistent values? > > > > I'm a bit unsure how to implement this for the ccw transport. Maybe a > > > new pair of ccws to read/write shared memory regions? > > > > Without knowing anything about CCW itself, I don't think you'd want > > to do calls to perform the reads/writes - remember these are entirely > > emulated devices, and the shared memory regions just correspond to > > memory regions in the hypervisor; so in most ways they just behave > > like a region of RAM. If the drivers can't treat them like RAM there's > > probably no point in using this feature in that environment. > > The main issue here is that s390 does not have memory mapped I/O -- > even PCI uses some specialized instructions. This means we need to > figure out how to model some stuff that Just Works on other platforms. > > So, basically there are two options: > - Have the device set aside a memory area; the host maps this into the > guest and the driver can access it. No notifications, only discovery > is needed. > - Have the device set aside a memory area; the driver can only access > this via special operations, which the host can trap. This needs two > more commands to be set aside, and any driver accesses need to be > forced through these commands (that's a bit like config space). > > If I've understood the intended usage correctly, we can use the simpler > first option. The drawback is that we can't add interception > possibilities (that we get via the second option) should we need them > later on. Won't write-protecting the memory make writes cause a trap that we can then handle? > > > > > But we'd also > > > need a mechanism to discover the ids of those shared memory regions, I > > > think. > > > > Yes, I'm assuming you'll need a call to enumerate them. > > Agreed. > > This publicly archived list offers a means to provide input to the > OASIS Virtual I/O Device (VIRTIO) TC. > > In order to verify user consent to the Feedback License terms and > to minimize spam in the list archive, subscription is required > before posting. > > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > List help: virtio-comment-help@lists.oasis-open.org > List archive: https://lists.oasis-open.org/archives/virtio-comment/ > Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf > List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists > Committee: https://www.oasis-open.org/committees/virtio/ > Join OASIS: https://www.oasis-open.org/join/ This publicly archived list offers a means to provide input to the OASIS Virtual I/O Device (VIRTIO) TC. 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