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diff --git a/shared-mem.tex b/shared-mem.tex new file mode 100644 index 0000000..6e6f6c4 --- /dev/null +++ b/shared-mem.tex @@ -0,0 +1,42 @@ +\section{Shared Memory Regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions} + +Shared memory regions are an additional facility +available to devices that need a region of memory that's +continuously shared between the device and the driver, rather +than passed between them in the way virtqueue elements are. + +Example uses include shared caches and version pools for versioned +data structures. + +The memory region is allocated by the device and presented to the +driver. Where the device is implemented in software on a host, +this arrangement allows the memory region to be allocated by +a library on the host, which the device may not have full control +over. + +A device may have multiple shared memory regions associated with +it. Each region has a \field{shmid} to identify it, the meaning +of which is device-specific. + +Enumeration and location of shared memory regions is performed +in a transport-specific way. + +Memory consistency rules vary depending on the region and the +device and they will be specified as required by each device. + +\subsection{Addressing within regions}\label{sec:Basic Facilities of a Virtio Device / Shared Memory Regions / Addressing within regions } + +References into shared memory regions are represented as offsets from +the beginning of the region instead of absolute memory addresses. +Offsets are used both for references between structures stored +within shared memory and for requests placed in virtqueues that +refer to shared memory. +The \field{shmid} may be explicit or may be inferred from the +context of the reference. + +\devicenormative{\subsection}{Shared Memory Regions}{Basic Facilities of a Virtio +Device / Shared Memory Regions} +Shared memory regions MUST NOT expose shared memory regions which +are used to control the operation of the device, nor to stream +data. + |