[PATCH] Add VIRTIO_RING_F_LARGE_INDIRECT_DESC

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

This new feature flag allows indirect descriptor tables to
exceed the queue size.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
---
Stefan noted that there should also be a numeric configuration field
reflecting a precise limit of indirect descriptors. The question is where
should that go to exactly? Some devices currently handle this in their
device specific configuration space. Wouldn't it make sense to handle that
in the common configuration space instead?
---
 content.tex    | 21 +++++++++++++++++++++
 split-ring.tex |  2 +-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/content.tex b/content.tex
index 5d112af..b42a26b 100644
--- a/content.tex
+++ b/content.tex
@@ -6693,6 +6693,27 @@ \chapter{Reserved Feature Bits}\label{sec:Reserved Feature Bits}
   transport specific.
   For more details about driver notifications over PCI see \ref{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Available Buffer Notifications}.
 
+  \item[VIRTIO_RING_F_LARGE_INDIRECT_DESC(40)] This feature indicates that the
+  amount of descriptors in an indirect descriptor table is allowed to exceed
+  the Queue Size.
+
+  If this feature bit is not negotiated, then a driver MUST NOT create a
+  descriptor chain longer than the Queue Size, which then also applies to
+  indirect descriptor tables as in \ref{sec:Basic Facilities of a Virtio
+  Device / Virtqueues / The Virtqueue Descriptor Table / Indirect
+  Descriptors}. Which means without this feature, the Queue Size limits
+  both the maximum amount of pending messages being emplaced in the vring,
+  as well as the actual bulk data size being transmitted.
+
+  With this feature enabled, the Queue Size only limits the maximum amount
+  of pending messages in the vring, but does not oppose a limit to the actual
+  bulk data size being transmitted. Decoupling these two configuration
+  parameters this way not only allows much larger bulk data being transferred
+  per message, but also avoids complicated synchronization mechanisms if
+  device only supports a very small amount of pending/active messages.
+  Due 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.
+
 \end{description}
 
 \drivernormative{\section}{Reserved Feature Bits}{Reserved Feature Bits}
diff --git a/split-ring.tex b/split-ring.tex
index bfef62d..5284635 100644
--- a/split-ring.tex
+++ b/split-ring.tex
@@ -269,7 +269,7 @@ \subsubsection{Indirect Descriptors}\label{sec:Basic Facilities of a Virtio Devi
 one table per descriptor).
 
 A driver MUST NOT create a descriptor chain longer than the Queue Size of
-the device.
+the device unless VIRTIO_RING_F_LARGE_INDIRECT_DESC has been negotiated.
 
 A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and VIRTQ_DESC_F_NEXT
 in \field{flags}.
-- 
2.20.1