Re: [Xen-devel] [PATCH 2/4] xen/vcpu: Document the xen_vcpu_info andxen_vcpu

[Konrad Rzeszutek Wilk]

On Tue, May 07, 2013 at 10:30:47AM +0100, Ian Campbell wrote:
> 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@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 @@
> >  
> >  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.