Re: [Xen-devel] [PATCH v4 2/4] x86/compat: Test both PV and PVH guests for compat mode

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 09/02/2015 04:08 AM, Jan Beulich wrote:
> > > > Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 09/02/15 2:55 AM >>>
> > On 08/27/2015 12:01 PM, Jan Beulich wrote:
> > > > > > On 13.08.15 at 20:12, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > > > @@ -777,7 +777,7 @@ int arch_set_info_guest(
> > > >    
> > > >        /* The context is a compat-mode one if the target domain is compat-mode;
> > > >         * we expect the tools to DTRT even in compat-mode callers. */
> > > > -    compat = is_pv_32bit_domain(d);
> > > > +    compat = is_pv_32bit_domain(d) || is_pvh_32bit_domain(d);
> > > I continue to think that this should include a v->domain ==
> > > current->domain check (to match behavior for HVM guests
> > > from the tool stack perspective). Having looked at patch 4, I also
> > > can't see how the tool stack is being made expect a non-native
> > > guest context record in the 32-bit PVH case (i.e. I'd appreciate
> > > if you could point out where that hides).
> > For vcpu 0 current->domain is dom0 so I am not sure how this check would
> > work.
> >
> > For a 32-bit PVH guest the toolstack will place data into
> > vcpu_guest_context_x86_32_t (in vcpu_x86_32()) and so the hypervisor,
> > knowing that the guest is a compat one (based on the test above), will
> > access appropriate fields.
> >
> > This is not how HVM guests are started --- "classic" PVH behaves very
> > much like a PV guest, unlike what we are doing with no-dm PVH.
> And I believe this to be wrong, and potentially getting in the way of the no-dm
> work - Roger?
>
> As to the reference to vcpu_x86_32() - by its name alone it is already clear
> that this is after the determination of what bit width a guest to deal with, and
> looking at xc_dom_32_pae I still can't see why PVH guests would (intentionally
> and legitimately) be treated like PV rather than HVM ones (not to speak of the
> fact that I don't think 32-bit PVH is in any way limited to PAE).
The purpose of this series is to get 32-bit guests to parity with 
classic PVH with minimal changes and then move on to no-dm. Not getting 
in the way of no-dm is obviously important but making classic behave 
like no-dm (which is, to certain extent, is what you are suggesting) is 
out of scope.
(I haven't considered non-PAE case, TBH)

-boris

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