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

[Ian Campbell <Ian.Campbell@citrix.com>]

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) {