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Re: [Xen-devel] how is start_info used by domU

  • To: Sandesh <sandesh.ahiremath@xxxxxxxxx>
  • From: "Grzegorz Miłoś" <gm281@xxxxxxxxx>
  • Date: Tue, 24 Jun 2008 19:32:21 +0100
  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Tue, 24 Jun 2008 11:32:42 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

> Hi,
>  I have a doubt regarding the way start_info fields are used by domU.
> I see that in start_info_x86_32(), most fields of start_info are getting
> initialized. I know that these info are used during the boot of guest
> OS, but iam not able to figure out as to how exactly this is achieved in
> the implementation?

First, Dom0 initialises start_info in DomU's memory. When it is
finished with the domain build, it hands the control over to DomU,
making sure that the location of the shared_info structure is know to
DomU's kernel. It does so, by saving pointer to start_info struct in
register %esi (look in vcpu_x86_32 defined in
tools/libxc/xc_dom_x86.c). When DomU kernel is executed, the first
thing it does, is to save the %esi to xen_start_info symbol (look in
startup_32 definied in arch/i386/kernel/head-xen.S). xen_start_info is
then available for later use.

> Another doubt i have is that the domain_builder executes vm_launch as
> its last step which makes setvcpucontext hypercall. After this when will
> it get loaded by the scheduler??

Python tools will take over (that's what executed domain_builder in
the first place) and unpause the domain after all the other bits and
pieces are set up. Xen will schedule DomU according to its built-in


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