Re: [Xen-devel] [PATCH v4 26/26] tools/libxc: Calculate xstate cpuid leaf from guest information

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 04/07/16 2:40 AM >>>
>On 07/04/2016 01:16, Jan Beulich wrote:
>>>>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 04/05/16 7:49 PM >>>
>>> There is no possible way of avoiding having a whitelist somewhere, which
>>> limits what Xen will tolerate supporting for the guest.
>> Right, but preferably in exactly one place. And imo that ought to be
>> info->xfeature_mask.
>
>info->xfeature_mask is actually Xen's limit, as obtained from
>XEN_DOMCTL_getvcpuextstate, so is an authoritative source of "the
>maximum Xen will support".
>
>However, the guest_xfeature_mask must be generated and used as this
>patch.  Without it, a domU will break if it migrates from a more capable
>xstate host to a less capable host, as using info->xfeature_mask alone
>leaks in state which should be levelled out.
>
>Currently upstream, heterogeneous migration of domains using xsave is
>broken if the domain first boots on the more-capable host.

I don't follow, I'm afraid: To me this looks like two separate things. One is to
suitably level the guest (via its config file), and the other is to not allow 
it to
use things the host doesn't support. If you want the guest to be migratable
to a less capable host, you need to configure the guest accordingly instead
of relying on a second instance of white listing.

Jan


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