Commit a520996a authored by Konrad Rzeszutek Wilk's avatar Konrad Rzeszutek Wilk
Browse files

xen/vcpu: Document the xen_vcpu_info and xen_vcpu 



They are important structures and it is not clear at first
look what they are for.

The xen_vcpu is a pointer. By default it points to the shared_info
structure (at the CPU offset location). However if the
VCPUOP_register_vcpu_info hypercall is implemented we can make the
xen_vcpu pointer point to a per-CPU location.

Acked-by: default avatarStefano Stabellini <stefano.stabellini@eu.citrix.com>
[v1: Added comments from Ian Campbell]
Signed-off-by: default avatarKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
parent 7f1fc268

arch/x86/xen/enlighten.c

+28 −1
Original line number Diff line number Diff line
@@ -85,7 +85,29 @@ 


EXPORT_SYMBOL_GPL(hypercall_page);



/*

 * Pointer to the xen_vcpu_info structure or

 * &HYPERVISOR_shared_info->vcpu_info[cpu]. See xen_hvm_init_shared_info

 * and xen_vcpu_setup for details. By default it points to share_info->vcpu_info

 * but if the hypervisor supports VCPUOP_register_vcpu_info then it can point

 * to xen_vcpu_info. The pointer is used in __xen_evtchn_do_upcall to

 * acknowledge pending events.

 * Also more subtly it is used by the patched version of irq enable/disable

 * e.g. xen_irq_enable_direct and xen_iret in PV mode.

 *

 * The desire to be able to do those mask/unmask operations as a single

 * instruction by using the per-cpu offset held in %gs is the real reason

 * vcpu info is in a per-cpu pointer and the original reason for this

 * hypercall.

 *

 */

DEFINE_PER_CPU(struct vcpu_info *, xen_vcpu);



/*

 * Per CPU pages used if hypervisor supports VCPUOP_register_vcpu_info

 * hypercall. This can be used both in PV and PVHVM mode. The structure

 * overrides the default per_cpu(xen_vcpu, cpu) value.

 */

DEFINE_PER_CPU(struct vcpu_info, xen_vcpu_info);



enum xen_domain_type xen_domain_type = XEN_NATIVE;
@@ -187,7 +209,12 @@ static void xen_vcpu_setup(int cpu) 


	/* Check to see if the hypervisor will put the vcpu_info

	   structure where we want it, which allows direct access via

	   a percpu-variable. */

	   a percpu-variable.

	   N.B. This hypercall can _only_ be called once per CPU. Subsequent

	   calls will error out with -EINVAL. This is due to the fact that

	   hypervisor has no unregister variant and this hypercall does not

	   allow to over-write info.mfn and info.offset.

	 */

	err = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info, cpu, &info);



	if (err) {

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