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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

>
> 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.

~Andrew

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