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    SubjectRe: [Xen-devel] [PATCH 2/4] xen/vcpu: Document the xen_vcpu_info and xen_vcpu
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    On Mon, 2013-05-06 at 14:04 +0100, Konrad Rzeszutek Wilk wrote:
    > 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@oracle.com>
    > ---
    > 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 @@
    >
    > 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.

    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 for the whole register_vcpu_info
    thing IIRC.

    > + */
    > 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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