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Subject: Re: [PATCH] virtio-transport: Clarify requirements
On Tue, Dec 05 2023, Viresh Kumar <viresh.kumar@linaro.org> wrote: > The virtio documentation currently doesn't define any generic > requirements that are applicable to all transports. They can be useful > while adding support for a new transport. > > This commit tries to define the same. Thank you for tackling this, albeit the devil's in the details :) > > Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org> > --- > content.tex | 48 ++++++++++++++++++++++++++++++++++++++++++++++-- > 1 file changed, 46 insertions(+), 2 deletions(-) > > diff --git a/content.tex b/content.tex > index 0a62dce5f65f..d4d5e7d7045b 100644 > --- a/content.tex > +++ b/content.tex > @@ -631,8 +631,52 @@ \section{Device Cleanup}\label{sec:General Initialization And Device Operation / > > \chapter{Virtio Transport Options}\label{sec:Virtio Transport Options} > > -Virtio can use various different buses, thus the standard is split > -into virtio general and bus-specific sections. > +The virtio devices are exposed to the guest as if they are physical > +devices using a specific transport method, like PCI, MMIO or Channel > +I/O. I'm not sure we can talk about "exposed to the guest" here, except as an example... maybe if we reword the whole paragraph (see my suggestion below.) > The transport methods define various aspects of the communication > +between the device and the driver, like device discovery, exchanging > +capabilities, interrupt handling, data transfer, etc.. Virtio can use > +various different buses, thus the standard is split into virtio general > +and bus-specific sections. I think we should concentrate on the transport being what links device and driver together... what about (reusing parts of your writeup): "Devices and drivers can use different transport methods to enable interaction, for example PCI, MMIO, or Channel I/O. The transport methods define various aspects of the communication between the device and the driver, like device discovery, exchanging capabilities, interrupt handling, data transfer, etc. For example, in a host/guest architecture, the host might expose a device to the guest on a PCI bus, and the guest will use a PCI-specific driver to interact with it. The standard is split into sections describing general virtio implementation and transport-specific sections." > + > +\section{Virtio Transport Requirements}\label{sec:Virtio Transport Options / Virtio Transport Requirements} > + > +\devicenormative{\subsection}{Virtio Transport Requirements}{Virtio Transport Options} I'm not sure we can introduce MUST (NOT) requirements for basic functionality after the spec has been published for quite a time already (although I'd assume every implementation is fulfilling the requirements anyway)... thoughts? > + > +The device MUST present each event, in a transport defined way, from the > +moment it takes place until the driver acknowledges the event. I don't believe "event" is well-defined here. > + > +The device MUST NOT access virtqueue's contents before the driver > +notifies that the queue is ready for access, in a transport defined way. > + > +The device MUST NOT access buffers on the virtqueue, after it has > +modified them and notified the driver about their availability. > + > +The device MUST reset the virtqueues if requested by the driver, in a > +transport defined way. Isn't all of this already defined in one place of the spec or another? > + > +\drivernormative{\subsection}{Virtio Transport Requirements}{Virtio Transport Options} > + > +The driver MUST NOT access guest memory locations outside what's made > +available by the device to the driver. I don't think that makes sense -- I'd assume most guest memory locations do not have anything to do with virtio, and we should try to avoid host/guest terminology. > + > +The driver MUST NOT write to the read-only memory area and MUST NOT read > +from the write-only memory area. Which memory areas does that refer to? Parts of the transport-specific data structures? > + > +The driver MUST acknowledge events presented by the device, as mandated > +by the transport. I don't think this is quite correct in the absolute -- for example, it should be fine to not acknowledge events if some overriding event comes along, or if the driver initiates a reset. > + > +The driver MUST NOT access virtqueue contents before the device notifies > +about the readiness of the same. > + > +The driver MUST NOT access buffers, after it has added them to the > +virtqueue and notified the device about their availability. The driver > +MAY access them after the device has processed them and notified the > +driver of their availability, in a transport defined way. > + > +The driver MAY ask the device to reset the virtqueues if, for example, > +the driver times out waiting for a notification from the device for a > +previously queued request. Again, I believe this has already been covered in the generic sections -- do we instead need to specify that a transport MUST provide a method to do xy? (or SHOULD, MAY, as applicable -- it would be good to list explicitly what is mandatory for a transport to implement, and what is optional.)
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