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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>
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1 changed files with 28 additions and 1 deletions
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@ -85,7 +85,29 @@
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EXPORT_SYMBOL_GPL(hypercall_page);
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/*
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* Pointer to the xen_vcpu_info structure or
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* &HYPERVISOR_shared_info->vcpu_info[cpu]. See xen_hvm_init_shared_info
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* and xen_vcpu_setup for details. By default it points to share_info->vcpu_info
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* but if the hypervisor supports VCPUOP_register_vcpu_info then it can point
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* to xen_vcpu_info. The pointer is used in __xen_evtchn_do_upcall to
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* acknowledge pending events.
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* Also more subtly it is used by the patched version of irq enable/disable
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* e.g. xen_irq_enable_direct and xen_iret in PV mode.
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*
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* The desire to be able to do those mask/unmask operations as a single
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* instruction by using the per-cpu offset held in %gs is the real reason
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* vcpu info is in a per-cpu pointer and the original reason for this
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* hypercall.
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*
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*/
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DEFINE_PER_CPU(struct vcpu_info *, xen_vcpu);
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/*
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* Per CPU pages used if hypervisor supports VCPUOP_register_vcpu_info
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* hypercall. This can be used both in PV and PVHVM mode. The structure
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* overrides the default per_cpu(xen_vcpu, cpu) value.
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*/
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DEFINE_PER_CPU(struct vcpu_info, xen_vcpu_info);
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enum xen_domain_type xen_domain_type = XEN_NATIVE;
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@ -187,7 +209,12 @@ static void xen_vcpu_setup(int cpu)
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/* Check to see if the hypervisor will put the vcpu_info
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structure where we want it, which allows direct access via
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a percpu-variable. */
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a percpu-variable.
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N.B. This hypercall can _only_ be called once per CPU. Subsequent
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calls will error out with -EINVAL. This is due to the fact that
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hypervisor has no unregister variant and this hypercall does not
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allow to over-write info.mfn and info.offset.
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*/
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err = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info, cpu, &info);
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if (err) {
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