Looking through the XenStore in Java Embed pdf417 in Java Looking through the XenStore 2d Data Matrix barcode for .NET

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8. Looking through the XenStore using none toproduce none with web,windows generate data matrix the request is none none added. Using the generic ring model, we would quickly run out of space in the rings. Instead, requests are enqueued in one ring, and responses in another, with an event (described in more detail in the next chapter) used to signal the arrival of a new response.

Requests sent to the back end are fairly similar conceptually to network packets. The fact that the structure used to represent them is called xsd sockmsg only helps to reinforce this parallel. The structure is actually the message header, rather than the entire message, and contains four elds, all of which are 32-bit integers, as shown here:.

Microsoft Office Word Website struct xsd { ui nt32 uint32 uint32 uint32 }; sockmsg t t t t type ; req id ; tx id ; len ;. The rst indica none none tes the type of the packet, and must be selected from an enumerated type declared in the same le. The second is a unique identi er. Any responses to this request will have the same req id value.

The tx id eld is used to indicate which transaction a request is part of. If a number of requests have to complete atomically, they can be grouped inside a pair of requests with the XS TRANSACTION START and XS TRANSACTION END types, respectively, and have the same transaction ID set for all of them. The back end should wait until the end of the transaction, lock any relevant data structures, and then execute the transaction in one go.

Finally, the len eld indicates the length of the body. After the header, the body of the message is generally a text string. The XenStore, for simplicity, is based around text strings.

If more than one string is required for a given request or response, they are separated by NULLs. For example, if you are writing a value, the path of the key and the associated value will both be provided as strings in the message body. Note that this imposes a practical limitation on the size of XenStore, because the length of a key, value and the message header must all be able to t in the request ring at once in order to set them.

In the current implementation, the rings are 1024 bytes each. The types of XenStore messages are all pre xed with XS . The simplest are XS READ and XS WRITE, responsible for reading and writing a key, respectively.

When reading a key, the XS READ command is used in the type eld of the message header, and the message body is set to a NULL-terminated string indicating the key. A response is then enqueued in the response ring with the same req id . If the type of the response is XS ERROR, the request failed.

In this case, the message body will contain the name of the error for example, ENOENT if the path did not exist. Note that a string is returned for an error, rather than a symbolic value..

8.4. Reading and Writing a Key If the read req none for none uest worked correctly, the message body of the response will contain the value associated with the key. A write request is handled in a similar way, although the request should contain two NULL-terminated (and, thus, NULL-separated) strings, the rst indicating the key path and the second indicating the value. Errors are reported in the same way, and a response of a type other than XS ERROR indicates success.

Keys paths are written in the same style as UNIX le paths, so /local/ domain/0/name is used to represent the path to the name of the dom0 guest. Each VM has its own subtree in /local/domain represented by its domain number, and all relative paths are assumed to start here. This allows a domain to check for devices without having to be aware of its domain number.

The path device/vbd/0, for example, always points to the rst virtual block device available to this domain. A list of all available device categories can be discovered by enumerating the keys in the device hierarchy, and the available devices of each type by enumerating keys in children. For example, the available virtual network interfaces can be discovered by enumerating the children of the device/vif key.

Enumerating keys is done using the XS DIRECTORY message type. This returns a list of NULL-terminated strings indicating the children of the given key. Note, again, that the children of any given key must t into the response ring in order for a guest to be able to enumerate them.

This is not typically a problem, because the size of keys is small, as is their number. If you are considering storing other information in the XenStore, however, keep these limitations in mind. The XenStore should be used as a mechanism for communicating small amounts of data.

It might be tempting to use it as a general purpose storage or communications system, but block devices, virtual interfaces, or shared memory pages are generally better for this kind of use..
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