Tuesday, June 26, 2007

Host Ordering Requirements: General Features

The HyperTransport specification breaks down the ordering rules governing transaction completion in the host system into a set of rules for ordered pairs of transactions. Depending on the request types and where the target locations are, the second request may be received but might have to wait to take effect in the host fabric until the first request reaches a specific point in completion called its ordering point. "Taking effect", in this case, means that a read request actually fetches data, a write request actually exposes new data, peer-to-peer requests are actually queued for reissue downstream, etc.

How read and write accesses originating in HyperTransport are handled depends on the type of space they target in the host system.

Cacheable address ranges have strongest ordering

Non-cacheable memory, I/O, and MMIO have weaker ordering

Interrupt and System Management Address ranges have special ordering

Two Ordering Points Are Defined
There are two ordering points (degrees of transaction completion) defined for the first transaction in an ordered pair; this information is used in determining whether the second request of the ordered pair may take effect or must wait. The ordering points are called Globally Ordered (GO) and Globally Visible (GV).

Globally Ordered (GO)
HyperTransport defines the globally ordered point for the first request as the point where it is guaranteed to be observed in the correct order (with respect to the second transaction) from any "observer". While the two transactions are guaranteed to complete in the proper order, they may not have actually done so yet. This means agents such as caches may not have been updated at this ordering point.

Globally Visible (GV)
HyperTransport defines the globally visible ordering point for the first request as the point where it is assured to be "visible" to all observers (CPUs, I/O devices, etc.). It also means that all side effects of the first request (cache transitions, etc.) have completed.

Note: If there are no "sideband" agents (caches, etc.), GO and GV are equivalent.

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