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authorKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>2013-05-05 08:51:47 -0400
committerKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>2013-05-07 10:05:34 -0400
commita520996ae2e2792e1f90b74e67c974120c8a3b83 (patch)
treef853e587a26c19707e797e967a8003a24fe544a5 /arch/x86
parent7f1fc268c47491fd5e63548f6415fc8604e13003 (diff)
downloadlinux-a520996ae2e2792e1f90b74e67c974120c8a3b83.tar.bz2
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>
Diffstat (limited to 'arch/x86')
-rw-r--r--arch/x86/xen/enlighten.c29
1 files changed, 28 insertions, 1 deletions
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 94a81f41e8a2..a2babdb13a26 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -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) {