Re: [PATCH 02/23] PVH xen: turn gdb_frames/gdt_ents into union.

[Mukesh Rathor <mukesh.rathor@oracle.com>]

On Tue, 6 Aug 2013 08:36:52 +0100
Ian Campbell <Ian.Campbell@citrix.com> wrote:

> On Mon, 2013-08-05 at 14:51 -0700, Mukesh Rathor wrote:
> > On Mon, 5 Aug 2013 16:55:22 +0100
> > George Dunlap <George.Dunlap@eu.citrix.com> wrote:
> > 
> > > On Sat, Jul 20, 2013 at 2:44 AM, Mukesh Rathor
> > > <mukesh.rathor@oracle.com> wrote:
> > > > Changes in V2:
> > > >   - Add __XEN_INTERFACE_VERSION__
> > > >
> > > >   Changes in V3:
> > > >     - Rename union to 'gdt' and rename field names.
> > > >
> > > >   Change in V9:
> > > >    - Update __XEN_LATEST_INTERFACE_VERSION__ to 0x00040400 for
> > > > compat.
> > > >
> > > > Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
> > > > Reviewed-by: Jan Beulich <jbeulich@suse.com>
> > > 
> > > One thing that's missing here is a description of *why* this
> > > change is being made.  Seeing that it's introducing a union isn't
> > > too difficult; harder is figuring out why that's necessary.
> > > 
> > > Presumably this is to more closely reflect how an HVM guest's GDT
> > > is stored -- i.e., in the guest's memory and checked by the
> > > hardware on use, rather than in Xen's memory, and checked by Xen
> > > on assignment?
> > 
> > Right. Unlike PV which passes it's gdt pages to xen to be
> > installed, a PVH only passes the GDT base and size, as it manages
> > its own GDT. I'll add more to the comments.
> 
> If the guest is managing its own gdt can't it also use lgdt
> instructions etc and expect the hypervisor to pull the GDT info out
> of the VMCS when it needs it?

That's what a PVH guest does during it's lifetime, ie, uses lgdt.
But like the comment says:

"The boot vcpu calls this to set some context for the non boot smp vcpu"

..we are doing this for jump starting a secondary vcpu to a very advanced 
known state. Once the vcpu is up and running, it just uses lgdt, loads
it's own selectors, etc....

thanks
mukesh