Re: [Xen-devel] RFC: very initial PVH design document

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Fri, 12 Sep 2014 16:38:20 -0400
Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:

> > ## SMP discover and bring up ##
> > 
> > The process of bringing up secondary CPUs is obviously different
> > from native, since PVH doesn't have a local APIC. The first thing
> > to do is to figure out how many vCPUs the guest has. This is done
> > using the `VCPUOP_is_up` hypercall, using for example this simple
> > loop:
> > 
> >     for (i = 0; i < MAXCPU; i++) {
> >         ret = HYPERVISOR_vcpu_op(VCPUOP_is_up, i, NULL);
> >         if (ret >= 0)
> >             /* vCPU#i is present */
> >     }
> > 
> > Note than when running as Dom0, the ACPI tables might report a
> > different number of available CPUs. This is because the value on
> > the ACPI tables is the number of physical CPUs the host has, and it
> > might bear no resemblance with the number of vCPUs Dom0 actually
> > has so it should be ignored.
> > 
> > In order to bring up the secondary vCPUs they must be configured
> > first. This is achieved using the `VCPUOP_initialise` hypercall. A
> > valid context has to be passed to the vCPU in order to boot. The
> > relevant fields for PVH guests are the following:
> > 
> >   * `flags`: contains VGCF_* flags (see `arch-x86/xen.h` public
> > header).
> >   * `user_regs`: struct that contains the register values that will
> > be set on the vCPU before booting. The most relevant ones are `rip`
> > and `rsp` in order to set the start address and the stack.
> 
> The OS can use 'rdi' and 'rsi' for their own purpose.
> 
> [Any other ones that are free to be used?]
> 

They are all. So, I would phrase it as:


`user_regs`: struct that contains the register values that will
       be set on the vCPU before booting. All GPRs are available to
       be set, however, the most relevant ones are `rip` and `rsp` in 
       order to set the start address and the stack.  Please note, all 
       selectors must be null.


In retrospect, maybe I should have tried harder to create a union
here, or even a new subcall for pvh, VCPUOP_initialise_pvh with
it's own struct.  Anyways... 

thanks,
Mukesh


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