Re: [Xen-devel] Status of 4.13

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.11.2019 16:20, George Dunlap wrote:
> 
> 
>> On Nov 21, 2019, at 8:41 AM, Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 21.11.2019 08:36, Jürgen Groß wrote:
>>> On 21.11.19 08:30, Steven Haigh wrote:
>>>> On 2019-11-21 17:05, Jürgen Groß wrote:
>>>>> Where do we stand with Xen 4.13 regarding blockers and related patches?
>>>>>
>>>>> 2. Ryzen/Rome failures with Windows guests:
>>>>>    What is the currently planned way to address the problem? Who is
>>>>>    working on that?
>>>>
>>>> A workaround was found by specifying cpuid values in the Windows VM 
>>>> config file.
>>>>
>>>> The workaround line is:
>>>> cpuid = [ "0x80000008:ecx=xxxxxxxxxxxxxxxx0100xxxxxxxxxxxx" ]
>>>>
>>>> It was suggested that this be documented - but no immediate action 
>>>> should be taken - with a view to correct this properly in 4.14.
>>>
>>> I'm aware of the suggestion, but not of any decision. :-)
>>
>> It was my understanding that we'd cap the 4-bit value to 7 for
>> the time being. I think George was planning to send a patch.
> 
> On that also, I’m aware of the suggestion, but not of any decision.
>  I don’t think I got much feedback, positive or negative, about the idea.
> 
> Suppose we implement the limit for 4.13. If someone runs Linux VMs
> on 4.12 a system with a hardware value of 7 for apic_id_size, the
> guests will see 8.  If they then migrate to 4.13, the value will
> magically change under their feet to 7.  Is that OK?

Let's look at the prereqs for running a Linux (or actually any) VM
on such hardware: At least on dual socket systems with such CPUs
Xen 4.12 wouldn't even boot. I don't know how wide a range of
single socket systems with these 64-code CPUs would exist or
appear down the road.

The workaround before our enabling of x2APIC mode for these boxes
was to disable SMT, which has the side effect of changing said
value to 6.

As to your actual question - as far as Linux goes, I don't think
they re-evaluate this CPUID leaf post boot. But I could be wrong
with this, and of course other OSes might behave differently.

Jan

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