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

On 07/04/16 01:56, Jan Beulich wrote:
>>>> 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.

Agreed, on all points.

But I assert that my change moves the code from being broken to working,
per the above description.

I have reworded several bits for v5 - perhaps that will make the patch
more clear.


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