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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

>
>>> Also the change here then results in Dom0 observing different
>>> behavior between faulting-capable and faulting-incapable hosts.
>>> I'm not convinced this is desirable.
>> I disagree.  Avoiding the leakage is very desirable moving forwards.
>>
>> Other side effects are that it makes PV and PVH dom0 functionally
>> identical WRT CPUID, and PV userspace (which, unlikely the kernel, tends
>> not to be Xen-aware) sees sensible information.
> I can see the upsides too, hence the "I'm not convinced" ...

So is that an ack or a nack?  I am afraid that this isn't very helpful.

~Andrew

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