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Xen 3.0 Virtualization Interface Guide
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A.1 Invoking Hypercalls

Hypercalls are invoked in a manner analogous to system calls in a conventional operating system; a software interrupt is issued which vectors to an entry point within Xen. On x86/32 machines the instruction required is int $82; the (real) IDT is setup so that this may only be issued from within ring 1. The particular hypercall to be invoked is contained in EAX -- a list mapping these values to symbolic hypercall names can be found in xen/include/public/xen.h.

On some occasions a set of hypercalls will be required to carry out a higher-level function; a good example is when a guest operating wishes to context switch to a new process which requires updating various privileged CPU state. As an optimization for these cases, there is a generic mechanism to issue a set of hypercalls as a batch:

multicall(void *call_list, int nr_calls)

Execute a series of hypervisor calls; nr_calls is the length of the array of multicall_entry_t structures pointed to be call_list. Each entry contains the hypercall operation code followed by up to 7 word-sized arguments.

Note that multicalls are provided purely as an optimization; there is no requirement to use them when first porting a guest operating system.

Xen 3.0 Virtualization Interface Guide
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  Published under the terms of the GNU General Public License Design by Interspire