Re: [Xen-devel] [PATCH] x86/vm_event: add gdtr_base to the vm_event structure

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 5/2/19 11:52 AM, Andrew Cooper wrote:
> On 02/05/2019 09:25, Razvan Cojocaru wrote:
> > On 5/2/19 2:57 AM, Tamas K Lengyel wrote:
> > > Receiving this register is useful for introspecting 32-bit Windows
> > > when the
> > > event being trapped happened while in ring3.
> > >
> > > Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> > > Cc: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
> > > Cc: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > > Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > > ---
> > >    xen/arch/x86/vm_event.c       | 5 +++++
> > >    xen/include/public/vm_event.h | 3 ++-
> > >    2 files changed, 7 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
> > > index 51c3493b1d..873788e32f 100644
> > > --- a/xen/arch/x86/vm_event.c
> > > +++ b/xen/arch/x86/vm_event.c
> > > @@ -179,6 +179,10 @@ static void vm_event_pack_segment_register(enum
> > > x86_segment segment,
> > >            reg->es_sel = seg.sel;
> > >            break;
> > >    +    case x86_seg_gdtr:
> > > +        reg->gdtr_base = seg.base;
> > > +        break;
> > Please also add limit, ar, sel, like the others do.
> In Xen, we model GDTR/IDTR just like all other segments in the segment
> cache for convenience/consistency, including faking up of some default
> attributes.
>
> However, there is no such thing as a selector or access rights for them,
> and the VMCS lacks the fields, while the VMCB marks the fields as
> reserved.   It is almost certainly not worth wasting the space in the ring.
Right, I got carried away there: I saw gdtr_limit and didn't check that 
if the rest is available.

Thanks,
Razvan

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