Re: [PATCH] x86/xsave: Remove xfeat_mask checking from validate_xstate()

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 30/05/2014 12:39, Jan Beulich wrote:
>>>> Andrew Cooper <andrew.cooper3@citrix.com> 05/30/14 10:39 AM >>>
>> validate_xsave() is call from the HVM and PV codepaths which load new vcpu
>> xsave state, usually as part of migration.  In both cases, this is the
>> xfeature_mask of the saving Xen rather than the restoring Xen.
>>
>> Given that the xsave state itself is checked for consistency and validity on
>> the current cpus, checking whether it was valid for the cpu before migration
>> is not interesting (or indeed relevant, as the error can't be distinguished
> >from the other validity checking).
>
> While I'm not entirely opposed to this, I'm also not fully convinced - the data
> being available, it can as well be used for additional sanity checking.
>
> Jan
>

What further sanity checking would be wanted/needed?

The sending Xen must have gotten this correct else it wouldn't have an 
xsave area to send in the first place.  If the receiving the Xen found 
parts it didn't like, the local validity checks would fail.

As far as I can see, the only case this might do something unexpected is 
if the individual xfeature_mask got changed on transit, at which point 
the receiving Xen would fail the xsave load, despite the xsave area 
being valid for the current cpu.

~Andrew