virtio — archive

[PATCH v10 00/10] Introduce device group and device management

From: Michael S. Tsirkin <[email protected]>
To: [email protected]
Date: Mon, 06 Mar 2023 22:00:00 -0000

On Mon, Mar 06, 2023 at 01:53:50PM +0000, David Edmondson wrote: > "Michael S. Tsirkin" <[email protected]> writes: > > > Feature negotiation forms the basis of forward compatibility > > guarantees of virtio but has never been properly documented. > > Do it now. > > > > Suggested-by: Halil Pasic <[email protected]> > > Signed-off-by: Michael S. Tsirkin <[email protected]> > > --- > > content.tex | 42 ++++++++++++++++++++++++++++++++++++++++++ > > 1 file changed, 42 insertions(+) > > > > diff --git a/content.tex b/content.tex > > index 0e474dd..0c2d917 100644 > > --- a/content.tex > > +++ b/content.tex > > @@ -114,21 +114,63 @@ \section{Feature Bits}\label{sec:Basic Facilities of a Virtio Device / Feature B > > In particular, new fields in the device configuration space are > > indicated by offering a new feature bit. > > > > +To keep the feature negotiation mechanism extensible, it is important > > +that devices \em{do not} offer any feature bits that they would not be > > +able to handle if the driver accepted them (even though drivers are not > > +supposed to accept them in the first place even if offered, according to > > +this version of the specification.) > > I find this (the bit in parenthesis) confusing. > > Why are drivers not supposed to accept features that they have been > offered, given that they can't know that the device cannot handle the > feature that it just offered? > > Is this alluding to the later section: > > > feature bits not described in this specification, reserved feature > > bits and feature bits reserved or not supported for the specific > > transport or the specific device type > > ? exactly. how would you put this better? given an example? > > Likewise, it is important that > > +drivers \em{do not} accept feature bits they do not know how to handle > > +(even though devices are not supposed to offer them in the first place, > > +according to this version of the specification.) The preferred way for > > +handling reserved and unexpected features is that the driver ignores > > +them. > > + > > +In particular, this is > > +especially important for features limited to specific transports, > > +as enabling these for more transports in future versions of the > > +specification is highly likely to require changing the behaviour > > +from drivers and devices. > > "require changed behaviour from drivers and devices". > > > Drivers and devices supporting > > +multiple transports need to carefully maintain per-transport > > +lists of allowed features. > > + > > \drivernormative{\subsection}{Feature Bits}{Basic Facilities of a Virtio Device / Feature Bits} > > The driver MUST NOT accept a feature which the device did not offer, > > and MUST NOT accept a feature which requires another feature which was > > not accepted. > > > > +The driver MUST validate the feature bits offered by the device. > > +The driver MUST ignore and MUST NOT accept any feature bit that is > > +\begin{itemize} > > +\item not described in this specification, > > +\item marked as reserved, > > +\item not supported for the specific transport, > > +\item not defined for the device type. > > +\end{itemize} > > + > > The driver SHOULD go into backwards compatibility mode > > if the device does not offer a feature it understands, otherwise MUST > > set the FAILED \field{device status} bit and cease initialization. > > > > +By contrast, the driver MUST NOT fail solely because a feature > > +it does not understand has been offered by the device. > > + > > \devicenormative{\subsection}{Feature Bits}{Basic Facilities of a Virtio Device / Feature Bits} > > The device MUST NOT offer a feature which requires another feature > > which was not offered. The device SHOULD accept any valid subset > > of features the driver accepts, otherwise it MUST fail to set the > > FEATURES_OK \field{device status} bit when the driver writes it. > > > > +The device MUST NOT offer feature bits corresponding to features > > +it would not support if accepted by the driver (even if the > > +driver is prohibited from accepting the feature bits by the > > +specification); for the sake of clarity, this refers to feature > > +bits not described in this specification, reserved feature bits > > +and feature bits reserved or not supported for the specific > > +transport or the specific device type, but this does not preclude > > +devices written to a future version of this specification from > > +offering such feature bits should such a specification have a > > +provision for devices to support the corresponding features. > > + > > If a device has successfully negotiated a set of features > > at least once (by accepting the FEATURES_OK \field{device > > status} bit during device initialization), then it SHOULD > > -- > > MST > > > > > > --------------------------------------------------------------------- > > To unsubscribe from this mail list, you must leave the OASIS TC that > > generates this mail. 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