From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: pasic@linux.ibm.com Date: Fri, 11 Jan 2019 16:29:47 +0100 From: Halil Pasic Subject: Re: [virtio-comment] [PATCH 1/3] shared memory: Define shared memory regions In-Reply-To: <20190111131540.12a8abca.cohuck@redhat.com> References: <20190111114200.10026-1-dgilbert@redhat.com> <20190111114200.10026-2-dgilbert@redhat.com> <20190111131540.12a8abca.cohuck@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-ID: <20190111162947.7a46e8df@oc2783563651> To: Cornelia Huck Cc: "Dr. David Alan Gilbert (git)" , virtio-comment@lists.oasis-open.org, stefanha@redhat.com List-ID: On Fri, 11 Jan 2019 13:15:40 +0100 Cornelia Huck wrote: > On Fri, 11 Jan 2019 11:41:58 +0000 > "Dr. David Alan Gilbert (git)" wrote: > > > From: "Dr. David Alan Gilbert" > > > > Define the requirements and idea behind shared memory regions. > > > > Signed-off-by: Dr. David Alan Gilbert > > --- > > content.tex | 3 +++ > > shared-mem.tex | 25 +++++++++++++++++++++++++ > > 2 files changed, 28 insertions(+) > > create mode 100644 shared-mem.tex > > > > diff --git a/content.tex b/content.tex > > index b101d1b..321a2f4 100644 > > --- a/content.tex > > +++ b/content.tex > > @@ -331,6 +331,9 @@ Virtqueue format, or both. > > \input{split-ring.tex} > > > > \input{packed-ring.tex} > > + > > +\input{shared-mem.tex} > > + > > \chapter{General Initialization And Device Operation}\label{sec:General Initialization And Device Operation} > > > > We start with an overview of device initialization, then expand on the > > diff --git a/shared-mem.tex b/shared-mem.tex > > new file mode 100644 > > index 0000000..6da249c > > --- /dev/null > > +++ b/shared-mem.tex > > @@ -0,0 +1,25 @@ > > +\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. > > + Kind of like the rings, just without any (common) pre-defined purpose and semantics? > > +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. > > + I will have to think about this paragraph some more... > > +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"? > > > + > > +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? > > 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? But we'd also > need a mechanism to discover the ids of those shared memory regions, I > think. > > Halil, do you have any thoughts? > I hope to develop more some. I've missed these discussions unfortunately, and the memory stuff is not my forte. But we do seem to need a mechanism to discover/expose (driver/device) these. Do we want to change the device initialization (3.1) subsection? I'm not sure if this shared memory region discovery is something that's supposed to be a part of the initialization. At the moment, I would guess is the device not supposed to be able to provide new regions at any time (as I don't see how the device is supposed to tell the driver: hey please re-do discovery). Regards, Halil