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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-08-06 at 14:50 +0100, George Dunlap wrote:
> On Tue, Aug 6, 2013 at 8:36 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> 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@xxxxxxxxxxxxx> wrote:
> >>
> >> > On Sat, Jul 20, 2013 at 2:44 AM, Mukesh Rathor
> >> > <mukesh.rathor@xxxxxxxxxx> 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@xxxxxxxxxx>
> >> > > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> >> >
> >> > 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?
> 
> If the toolstack wants to read the state, maybe?

"when it needs to" would include when the toolstack uses whichever
domctl queries this stuff (if there even is one)

Ian.


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