Re: [PATCH v9 05/10] pci: add admin vq registers to virtio over pci

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Nov 24, 2022 at 03:46:00PM +0800, Jason Wang wrote:
> On Thu, Nov 24, 2022 at 3:14 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Thu, Nov 24, 2022 at 02:00:54PM +0800, Jason Wang wrote:
> > > On Thu, Nov 24, 2022 at 5:08 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > Add new registers to the PCI common configuration structure.
> > > >
> > > > These registers will be used for querying the indices of the admin
> > > > virtqueues of the owner device. To configure, reset or enable the admin
> > > > virtqueues, the driver should follow existing queue configuration/setup
> > > > sequence.
> > > >
> > > > Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > ---
> > > >  content.tex | 34 ++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 34 insertions(+)
> > > >
> > > > diff --git a/content.tex b/content.tex
> > > > index 04235da..aa68926 100644
> > > > --- a/content.tex
> > > > +++ b/content.tex
> > > > @@ -946,6 +946,10 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> > > >          le64 queue_device;              /* read-write */
> > > >          le16 queue_notify_data;         /* read-only for driver */
> > > >          le16 queue_reset;               /* read-write */
> > > > +
> > > > +        /* About the administration virtqueue. */
> > > > +        le16 admin_queue_index;         /* read-only for driver */
> > > > +        le16 admin_queue_num;         /* read-only for driver */
> > >
> > > If we are adding more queue related functionality, it would be placed
> > > under the above two fields which seems not elegant. I wonder if a new
> > > structure is needed here.
> > >
> > > Thanks
> >
> > I don't expect more, if that happens we can always refactor it.
> 
> I may miss something but it should be a part of ABI. How could we
> refactor it without breaking ABI?
> 
> Thanks

         /* About the administration virtqueue. */
-        le16 admin_queue_index;         /* read-only for driver */
-        le16 admin_queue_num;         /* read-only for driver */
+        struct { /* read-only for driver */
+        	le16 index;
+        	le16 num;
+	 } admin_queue;

does not affect the ABI. But the order of things makes it less readable.

We would also have to change references to admin_queue_index with
admin_queue.index and explain the "." notation in introduction.


All doable but this patchset is large enough as it is.



> > Generally this is not C, no one compiles this directly.
> > So we do not need to add structures unless
> > they are referred to in spec text.
> >
> > > >  };
> > > >  \end{lstlisting}
> > > >
> > > > @@ -1031,6 +1035,19 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> > > >          This field exists only if VIRTIO_F_RING_RESET has been
> > > >          negotiated. (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
> > > >
> > > > +\item[\field{admin_queue_index}]
> > > > +        The device uses this to report the index of the first administration virtqueue.
> > > > +        This field is valid only if VIRTIO_F_ADMIN_VQ has been negotiated.
> > > > +\item[\field{admin_queue_num}]
> > > > +       The device uses this to report the number of the
> > > > +       supported administration virtqueues.  Virtqueues with index
> > > > +       between \field{admin_queue_index} and (\field{admin_queue_index} +
> > > > +       \field{admin_queue_num}) inclusive serve as administration
> > > > +       virtqueues.
> > > > +       Thus the number of administration virtqueues equals
> > > > +       (\field{admin_queue_num} + 1).
> > > > +       This field is valid only if VIRTIO_F_ADMIN_VQ has been
> > > > +       negotiated.
> > > >  \end{description}
> > > >
> > > >  \devicenormative{\paragraph}{Common configuration structure layout}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Common configuration structure layout}
> > > > @@ -1117,6 +1134,14 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
> > > >  were used before the queue reset.
> > > >  (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
> > > >
> > > > +If VIRTIO_F_ADMIN_VQ has been negotiated, and if the driver
> > > > +configures any administration virtqueues, the driver MUST
> > > > +configure the administration virtqueues using the index
> > > > +in the range \field{admin_queue_index} to
> > > > +\field{admin_queue_index} + \field{admin_queue_num} inclusive.
> > > > +The driver MAY configure less administration virtqueues than
> > > > +supported by the device.
> > > > +
> > > >  \subsubsection{Notification structure layout}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Notification capability}
> > > >
> > > >  The notification location is found using the VIRTIO_PCI_CAP_NOTIFY_CFG
> > > > @@ -6992,6 +7017,15 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
> > > >
> > > >    \item[VIRTIO_F_ADMIN_VQ(41)] This feature indicates that the device exposes one or more
> > > >    administration virtqueues.
> > > > +  At the moment this feature is only supported for devices using
> > > > +  \ref{sec:Virtio Transport Options / Virtio Over PCI
> > > > +         Bus}~\nameref{sec:Virtio Transport Options / Virtio Over PCI Bus}
> > > > +         as the transport and is reserved for future use for
> > > > +         devices using other transports (see
> > > > +         \ref{drivernormative:Basic Facilities of a Virtio Device / Feature Bits}
> > > > +       and
> > > > +       \ref{devicenormative:Basic Facilities of a Virtio Device / Feature Bits} for
> > > > +       handling features reserved for future use.
> > > >
> > > >  \end{description}
> > > >
> > > > --
> > > > MST
> > > >
> >