Re: [Xen-devel] [V10 PATCH 09/23] PVH xen: introduce pvh_set_vcpu_info() and vmx_pvh_set_vcpu_info()

[Tim Deegan <tim@xxxxxxx>]

At 12:04 +0100 on 12 Aug (1376309065), Jan Beulich wrote:
> >>> On 12.08.13 at 12:24, Tim Deegan <tim@xxxxxxx> wrote:
> > At 08:54 +0100 on 12 Aug (1376297674), Jan Beulich wrote:
> >> >>> On 10.08.13 at 01:41, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:
> >> > Since we are loading gdtr and selectors cs/ds/ss, we should also load
> >> > the hidden fields for cs/ds/ss. That IMO is plenty enough support for
> >> > the vcpu to come up, and the vcpu itself can then load ldtr, fs base, gs
> >> > base, etc first thing in it's HVM container. What do you all think?
> >> 
> >> If you implement loading the hidden fields of CS, then doing the
> >> same for the LDT shouldn't be that much more code (and if you
> >> permit a non-null LDT selector, then having it in place would even
> >> be a requirement before validating the CS selector). But again,
> >> I had already indicated that I'd be fine with requiring the state to
> >> be truly minimal: CS -> flat 64-bit code descriptor, SS, DS, ES, FS
> >> and GS holding null selectors. And CS descriptor validation done
> >> only in debug mode.
> > 
> > If you're going that way, please go the whole hog:
> >  - _all_ of cs/ss/ds/es/fs/gs arguments required to be null
> >    (and so documented, and enforced).
> >  - GDT base/limit loaded from the args.
> >  - LDT base/limit args required (documented, enforced) to be zero.
> >  - Guest launches with a flat 32/64bit segments set up in the
> >    hidden part of all segments (or I guess on 32-bit you could have all
> >    but CS invalid).  Then it can load its real segment state after boot.
> > 
> > That way we don't have the weird constraints on the layout/contents
> > of the guest's GDT or on its segment descriptors.
> > 
> > Would that be OK?
> 
> I don't think CS = null is valid.

Ah, you're right, that's explicilty disallowed. :(  Xen could set any
non-null descriptor (still requiring the caller to specify null
descriptors and documenting that the descriptor registers will be
undefined until the guest loads them on the new vcpu).

If we really require the selectors to match the GDT contents then we
either have to constrain the selector/GDT contents (a horrible
interface) or properly emulate the loads (which surely we already have
code in Xen to do).

Tim.

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