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Diffstat (limited to 'content.tex')
| -rw-r--r-- | content.tex | 41 |
1 files 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} |