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Subject: Re: [virtio-dev] Re: [virtio-comment] [PATCH v4 1/3] shared memory: Define shared memory regions
* Michael S. Tsirkin (mst@redhat.com) wrote:
> 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 .
How about:
The memory region is allocated by the device and presented to the
driver. Where the device is implemented in software on a host,
this arrangement allows the memory region to be allocated by
a library on the host, which the device may not have full control
over.
Dave
>
> > > > +
> > > > +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.
> > > >
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> > > >
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> > --
> > 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.
> >
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> >
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
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