Re: [virtio-comment] [PATCH v4 1/3] shared memory: Define shared memory regions

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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.

> > +
> > +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

> 
> > +
> > -- 
> > 2.20.1
> > 
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK