[PATCH] legacy: clean up virtqueue layout definitions

[Cornelia Huck <cornelia.huck@de.ibm.com>]

Generalize "Legacy Interfaces: A Note on Virtqueue Layout" to allow
for different alignment requirements. Have pci and ccw refer to that
section for legacy devices. Remove the double definition of virtqueue
alignment (which referred to legacy, but was not tagged as such) from
the ccw section.

VIRTIO-129

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
---
 content.tex |   41 ++++++++++-------------------------------
 1 file changed, 10 insertions(+), 31 deletions(-)

diff --git a/content.tex b/content.tex
index 1efdcc8..6a8638b 100644
--- a/content.tex
+++ b/content.tex
@@ -288,7 +288,8 @@ For Legacy Interfaces, several additional
 restrictions are placed on the virtqueue layout:
 
 Each virtqueue occupies two or more physically-contiguous pages
-(usually defined as 4096 bytes, but depending on the transport)
+(usually defined as 4096 bytes, but depending on the transport;
+henceforth referred to as Queue Align)
 and consists of three parts:
 
 \begin{tabular}{|l|l|l|}
@@ -301,11 +302,12 @@ The bus-specific Queue Size field controls the total number of bytes
 for the virtqueue.
 When using the legacy interface, the transitional
 driver MUST retrieve the Queue Size field from the device
-and MUST allocate the total number of bytes for the virtuqueue
-according to the following formula:
+and MUST allocate the total number of bytes for the virtqueue
+according to the following formula (Queue Align given in qalign and
+Queue Size given in qsz):
 
 \begin{lstlisting}
-#define ALIGN(x) (((x) + PAGE_SIZE) & ~PAGE_SIZE)
+#define ALIGN(x) (((x) + qalign) & ~qalign)
 static inline unsigned virtq_size(unsigned int qsz)
 {
      return ALIGN(sizeof(struct virtq_desc)*qsz + sizeof(u16)*(3 + qsz))
@@ -326,7 +328,7 @@ struct virtq {
         // A ring of available descriptor heads with free-running index.
         struct virtq_avail avail;
 
-        // Padding to the next PAGE_SIZE boundary.
+        // Padding to the next Queue Align boundary.
         u8 pad[ Padding ];
 
         // A ring of used descriptor heads with free-running index.
@@ -1761,8 +1763,8 @@ The driver typically does this as follows, for each virtqueue a device has:
 \end{enumerate}
 
 \subparagraph{Legacy Interface: A Note on Virtqueue Configuration}\label{sec:Virtio Transport Options / Virtio Over PCI Bus / PCI-specific Initialization And Device Operation / Device Initialization / Virtqueue Configuration / Legacy Interface: A Note on Virtqueue Configuration}
-When using the legacy interface, the page size for a virtqueue on a PCI virtio
-device is defined as 4096 bytes.  Driver writes the physical address, divided
+When using the legacy interface, the queue layout follows \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Legacy Interfaces: A Note on Virtqueue Layout}~\nameref{sec:Basic Facilities of a Virtio Device / Virtqueues / Legacy Interfaces: A Note on Virtqueue Layout} with an alignment of 4096.
+Driver writes the physical address, divided
 by 4096 to the Queue Address field\footnote{The 4096 is based on the x86 page size, but it's also large
 enough to ensure that the separate parts of the virtqueue are on
 separate cache lines.
@@ -2585,30 +2587,7 @@ struct vq_info_block_legacy {
 \end{lstlisting}
 
 \field{queue} contains the guest address for queue \field{index}, \field{num} the number of buffers
-and \field{align} the alignment.
-
-\subsubsection{Virtqueue Layout}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Virtqueue Layout}
-
-The virtqueue is physically contiguous, with padding added to make the
-used ring meet the align value:
-
-\begin{tabular}{|l|l|l|}
-\hline
-Descriptor Table & Available Ring (\ldots padding\ldots) & Used Ring \\
-\hline
-\end{tabular}
-
-The calculation for total size is as follows:
-
-\begin{lstlisting}
-#define ALIGN(x) (((x) + align) & ~align)
-static inline unsigned virtq_size(unsigned int num)
-{
-     return ALIGN(sizeof(struct virtq_desc)*num
-                  + sizeof(u16)*(3 + num))
-          + ALIGN(sizeof(u16)*3 + sizeof(struct virtq_used_elem)*num);
-}
-\end{lstlisting}
+and \field{align} the alignment. The queue layout follows \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Legacy Interfaces: A Note on Virtqueue Layout}~\nameref{sec:Basic Facilities of a Virtio Device / Virtqueues / Legacy Interfaces: A Note on Virtqueue Layout}.
 
 \subsubsection{Communicating Status Information}\label{sec:Virtio Transport Options / Virtio over channel I/O / Device Initialization / Communicating Status Information}
 
-- 
1.7.9.5