Re: [Xen-devel] [PATCH 10/10] x86/cpuid: Always enable faulting for the control domain

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Mar 13, 2017 at 05:48:44AM -0600, Jan Beulich wrote:
> >>> On 10.03.17 at 18:10, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 28/02/17 09:31, Jan Beulich wrote:
> >>>>> On 27.02.17 at 16:10, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>> On 22/02/17 10:10, Jan Beulich wrote:
> >>>>>>> On 22.02.17 at 11:00, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>>>> On 22/02/17 09:23, Jan Beulich wrote:
> >>>>>>>>> On 20.02.17 at 12:00, <andrew.cooper3@xxxxxxxxxx> wrote:
> >>>>>>> The domain builder in libxc no longer depends on leaked CPUID 
> >>>>>>> information to
> >>>>>>> properly construct HVM domains.  Remove the control domain exclusion.
> >>>>>> Am I missing some intermediate step? As long as there's a raw
> >>>>>> CPUID invocation in xc_cpuid_x86.c (which is still there in staging
> >>>>>> and I don't recall this series removing it) it at least _feels_ unsafe.
> >>>>> Strictly speaking, the domain builder part of this was completed after
> >>>>> my xsave adjustments.  All the guest-type-dependent information now
> >>>>> comes from non-cpuid sources in libxc, or Xen ignores the toolstack
> >>>>> values and recalculates information itself.
> >>>>>
> >>>>> However, until the Intel leaves were complete, dom0 had a hard time
> >>>>> booting with this change as there were no toolstack-provided policy and
> >>>>> no leakage from hardware.
> >>>> So what are the CPUID uses in libxc then needed for at this point?
> >>>> Could they be removed in a prereq patch to make clear all needed
> >>>> information is now being obtained via hypercalls?
> >>> I'd prefer to defer that work.  The next chunk of CPUID work is going to
> >>> be redesigning and reimplementing the hypervisor/libxc interface, and
> >>> all cpuid() calls in libxc will fall out there, but its not a trivial
> >>> set of changes to make.
> >> With that, could you live with deferring the patch here until then?
> > 
> > We currently have a lot of dom0 implicit dependencies on leaked CPUID
> > state into PV dom0.
> > 
> > With this series, I believe I have identified all leaked dependencies,
> > and I really want to prevent is introducing any new implicit dependences
> > accidentally.
> 
> I can certainly understand this, but the state libxc code is in then
> makes this a rather implicit thing, instead of being fully explicit. I
> think I'd like to have another (tools or REST) maintainer voice a 3rd
> opinion. Extending Cc list ...
> 

I'm not sure I follow the implicit vs explicit bit.

Wei.

> Jan
> 

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