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[Xen-devel] [PATCH 2/4] 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.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
 arch/x86/xen/enlighten.c | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 94a81f4..9b34475 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -85,7 +85,21 @@
+ * 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 
+ * 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.
+ */
 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 +201,12 @@ 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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