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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

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?

 -George

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