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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: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[v1: Added comments from Ian Campbell]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
This commit is contained in:
Konrad Rzeszutek Wilk 2013-05-05 08:51:47 -04:00
parent 7f1fc268c4
commit a520996ae2
1 changed files with 28 additions and 1 deletions
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29
arch/x86/xen/enlighten.c
View File

@ -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) {