Re: [Xen-devel] CPUID improvements (phase 2) Design Doc

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.11.16 at 19:36, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 08/11/16 16:32, Jan Beulich wrote:
>>>>> On 08.11.16 at 16:35, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> Please find inline the design doc for further CPUID improvements, planned 
>>> for
>>> development during the 4.9 timeframe.
>> Looks good, just a couple of minor remarks.
>>
>>> ## Changes in hypercall behaviour
>>>
>>> During domain construction, some pieces of information critical to the
>>> determination of the domains maximum acceptable CPUID policy are available
>>> right from the very start (Most notably, the HVM and HAP flags from the
>>> `XEN_DOMCTL_createdomain`).
>>>
>>> However, some other parameters are not available at convenient points.
>>>
>>> 1.  The disable flag from `XEN_DOMCTL_disable_migrate` is used to set
>>>     `d->disable_migrate`, whose only purpose is to avoid the unconditional
>>>     clobbering of the Invariant TSC flag.  This flag cannot even be queried 
>>> by
>>>     the toolstack once set.
>>>
>>>     There are other facilities which should be restricted based on whether a
>>>     VM might migrate or not.  (e.g. The use of LBR, whose record format is
>>>     hardware specific.)
>> Not really - the LBR format only limits the set of hosts the VM can
>> migrate to. I.e. this is just like a CPUID flag which needs to be set
>> on the target host in order for the VM to be permitted to migrate
>> there.
> 
> It is more complicated than that.  The LBR format also depends on
> whether TSX is enabled or not, which on Haswell-WS CPUs depends on
> whether hyperthreading is enabled.

Yes, but is this relevant? It's still only a value (identifying the format)
which needs to match between source and destination hosts. I.e. not
different from individual feature bits, just that here we're dealing with
a multi-bit entity.

Jan


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