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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.

But we don't know driver is guest and device is host.

This is why I suggested that it's an example:
for example if driver is in guest and device is in host software
then another host library can map it.


> > > +
> > > +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

I think writes and read to these regions must not have
side effects. Is this what you are trying to say?

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


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