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Subject: Re: [virtio-dev] Re: [PATCH v2 1/3] shared memory: Define shared memory regions
* Cornelia Huck (cohuck@redhat.com) wrote: > On Tue, 26 Feb 2019 15:11:34 +0000 > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote: > > > * Cornelia Huck (cohuck@redhat.com) wrote: > > > On Fri, 22 Feb 2019 11:54:31 +0000 > > > "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> 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> > > > > --- > > > > content.tex | 2 ++ > > > > shared-mem.tex | 33 +++++++++++++++++++++++++++++++++ > > > > 2 files changed, 35 insertions(+) > > > > create mode 100644 shared-mem.tex > > > > > > > > diff --git a/content.tex b/content.tex > > > > index 836ee52..3dd504c 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..85b0c55 > > > > --- /dev/null > > > > +++ b/shared-mem.tex > > > > @@ -0,0 +1,33 @@ > > > > +\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. > > > > + > > > > +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. > > > > > > This explanation looks good to me. > > > > > > > + > > > > +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. > > > > > > The 'MUST NOT' makes it look like a normative statement; however, this > > > is more like design advice? > > > > Hmm I'm not sure - the intention of this line is to try and stop people > > using it as a hack to avoid standardising protocol for no good reason. > > I've looked through the spec again and it seems the best place for this > would indeed be a device-normative section for shared regions; it would > imply that a device needs to comply with this statement, or it is not > conformant. > > It would need some tweaking, however. Perhaps something like the > following: > > "The device MUST NOT expose shared memory regions which are used to > control the operation of the device, or to stream data." > Not sure where to put the pointer to use virtqueues for that. Thanks I've turned that into a device normative section and dropped the explicit mention of virtqueues. > > > > > > > + > > > > +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. > > > > + > > > > +Memory consistency rules vary depending on the region and the > > > > +device. Devices MUST define the required behaviour for each > > > > +region. > > > > > > Same here. > > > > OK, so should I reword this? > > Maybe put into the device normative section: > > "A device MUST define the required behaviour for each region." > > But I'm not sure whether this is a good normative statement... it's > more that the regions have specific requirements, which both the device > and the driver need to fulfill, and those requirements obviously need > to be put down somewhere. > > > > > > > + > > > > +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. > > > > + > > > > > > For that high-level overview, I'm not sure if any normative statements > > > are needed/wanted, or whether those should be confined to the individual > > > transport or device type definitions... > > > > I think this one is stronger than the previous two; if people start > > passing GPA/HVA in the underlying structures things are going to get > > messy. > > What about > > "The device MUST handle any access to the memory region as addressed > relatively to the beginning of the region, regardless whether it > accesses the region itself or the driver does so." > > "The driver MUST address any particular region relatively to the start > of that region." > > for device and driver normative statements, respectively. Better > wordings welcome. What I've done here is turn this into just a subsection describing how it should work and stopped using the magic MUST. It's not really a requirement on the device - it's more an indication of how to design a device. Dave > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org > For additional commands, e-mail: virtio-dev-help@lists.oasis-open.org > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
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