[PATCH v3 2/4] Add PCI configuration field "queue_indirect_size"

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

This new PCI configuration field allows to negotiate a more fine
graded maximum lenght of indirect descriptor chains.

Fixes: https://github.com/oasis-tcs/virtio-spec/issues/122
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 content.tex | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/content.tex b/content.tex
index 685525d..5162e2b 100644
--- a/content.tex
+++ b/content.tex
@@ -902,6 +902,7 @@ \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 */
+        le32 queue_indirect_size;       /* read-write */
 };
 \end{lstlisting}
 
@@ -987,6 +988,17 @@ \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{queue_indirect_size}]
+        This field is used to negotiate the maximum number of descriptors per
+        indirect descriptor table as in \ref{sec:Basic Facilities of a Virtio
+        Device / Virtqueues / The Virtqueue Descriptor Table / Indirect
+        Descriptors} if and only if the VIRTIO_RING_F_INDIRECT_SIZE feature has
+        been negotiated.
+
+        The device specifies its maximum number of descriptors per indirect
+        descriptor table. If the driver requires fewer descriptors, it writes
+        its lower value to inform the device of the reduced resource
+        requirements.
 \end{description}
 
 \devicenormative{\paragraph}{Common configuration structure layout}{Virtio Transport Options / Virtio Over PCI Bus / PCI Device Layout / Common configuration structure layout}
@@ -1081,6 +1093,12 @@ \subsubsection{Common configuration structure layout}\label{sec:Virtio Transport
 values to different values than those that were used before the queue reset.
 (see \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Virtqueue Reset}).
 
+The driver SHOULD write to \field{queue_indirect_size} if its maximum number of
+descriptors per vring slot is lower than that reported by the device.
+
+The driver MUST NOT write a higher value to \field{queue_indirect_size} than the
+one it reads from 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
@@ -6870,10 +6888,10 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
   to the 16-bit size of a descriptor's "next" field there is still an absolute
   limit of $2^{16}$ descriptors per indirect descriptor table. However the
   actual maximum amount supported by either device or driver might be less,
-  and therefore the bus specific Queue Indirect Size value MUST additionally
-  be negotiated if VIRTIO_RING_F_INDIRECT_SIZE was negotiated to subsequently
-  negotiate the actual amount of maximum indirect descriptors supported
-  by both sides.
+  and therefore the transport specific Queue Indirect Size value MUST
+  additionally be negotiated if VIRTIO_RING_F_INDIRECT_SIZE was negotiated to
+  subsequently negotiate the actual amount of maximum indirect descriptors
+  supported by both sides.
 
 \end{description}
 
-- 
2.30.2