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Subject: Re: [virtio-comment] [PATCH v4 1/3] shared memory: Define shared memory regions
On Wed, Jun 19, 2019 at 05:06:04PM +0100, Dr. David Alan Gilbert wrote: > * Michael S. Tsirkin (mst@redhat.com) wrote: > > On Wed, Mar 20, 2019 at 12:07:38PM +0000, Dr. David Alan Gilbert (git) wrote: > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com> > > > > > > Define the requirements and idea behind shared memory regions. > > > > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> > > > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> > > > --- > > > conformance.tex | 1 + > > > content.tex | 2 ++ > > > shared-mem.tex | 40 ++++++++++++++++++++++++++++++++++++++++ > > > 3 files changed, 43 insertions(+) > > > create mode 100644 shared-mem.tex > > > > > > diff --git a/conformance.tex b/conformance.tex > > > index ad7e82e..f6c0d50 100644 > > > --- a/conformance.tex > > > +++ b/conformance.tex > > > @@ -195,6 +195,7 @@ A device MUST conform to the following normative statements: > > > \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / Used Buffer Notification Suppression} > > > \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / The Virtqueue Used Ring} > > > \item \ref{devicenormative:Basic Facilities of a Virtio Device / Virtqueues / Available Buffer Notification Suppression} > > > +\item \ref{devicenormative:Basic Facilities of a Virtio Device / Shared Memory Regions} > > > \item \ref{devicenormative:Reserved Feature Bits} > > > \end{itemize} > > > > > > diff --git a/content.tex b/content.tex > > > index ede0ef6..8cd1a38 100644 > > > --- a/content.tex > > > +++ b/content.tex > > > @@ -371,6 +371,8 @@ making any more buffers available. When VIRTIO_F_NOTIFICATION_DATA > > > has been negotiated, these notifications would then have > > > identical \field{next_off} and \field{next_wrap} values. > > > > > > +\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..6dc506a > > > --- /dev/null > > > +++ b/shared-mem.tex > > > @@ -0,0 +1,40 @@ > > > +\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. > > > > Please rewrite host->device, guest->driver > > Done. > > > > + > > > +Example uses include shared caches and version pools for versioned > > > +data structures. > > > + > > > +The region is chosen by the host and presented to the guest, as > > > +such it is useful in situations where the memory is accessed on > > > +the host by other libraries that can't safely access guest RAM. > > > > I'm not sure what does above mean in context of virtio. > > Is this an example? > > So maybe: > > For example, with device being part of a > > hypervisor and driver being part of a VM guest > > etc etc > > It means the memory can be allocated by another library in the host > and then passed into the guest; the alternative is where the guest > gives a pointer into the guests own RAM. OK so let's say this like this: memory is allocated by the device as opposed to by driver. As such this allows configurations where access to guest memory is limited. For example and so . > > > + > > > +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 and mechanism. > > > > Let's just say in a transport-specific way. > > Done. > > > > + > > > +Memory consistency rules vary depending on the region and the > > > +device and they will be specified as required by each device. > > > + > > > +\subsection{Addressing within regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions / Addressing within regions } > > > + > > > +References into shared memory regions are represented as offsets from > > > +the beginning of the region instead of absolute memory addresses. > > > +Offsets are used both for references between structures stored > > > +within shared memory and for requests placed in virtqueues that > > > +refer to shared memory. > > > +The \field{shmid} may be explicit or may be inferred from the > > > +context of the reference. > > > + > > > +\devicenormative{\subsection}{Shared Memory Regions}{Basic Facilities of a Virtio > > > +Device / Shared Memory Regions} > > > +Shared memory regions MUST NOT expose shared memory regions which > > > +are used to control the operation of the device, nor to stream > > > +data. > > > > regions must not expose regions. confused. > > typo? > > > > Also, what does stream data mean? > > I'm trying to find a way to say that people shouldn't side-step virtio > queues by putting everything in a big blob of shared memory. > > Better wording welcome. > > Dave The way to improve wording would be by specifying what are the disadvantages of storing data in shared memory, and when it should be avoided. > > > > > + > > > -- > > > 2.20.1 > > > > > > > > > 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/ > -- > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK > > 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/
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