Re: [Xen-devel] [PATCH v11 6/7] microcode: rendezvous CPUs in NMI handler and load ucode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.09.2019 15:53, Chao Gao wrote:
> On Fri, Sep 27, 2019 at 12:19:22PM +0200, Jan Beulich wrote:
>> On 26.09.2019 15:53, Chao Gao wrote:
>>> @@ -420,14 +498,23 @@ static int control_thread_fn(const struct 
>>> microcode_patch *patch)
>>>          return ret;
>>>      }
>>>  
>>> -    /* Let primary threads load the given ucode update */
>>> -    set_state(LOADING_ENTER);
>>> -
>>> +    /* Control thread loads ucode first while others are in NMI handler. */
>>>      ret = microcode_ops->apply_microcode(patch);
>>>      if ( !ret )
>>>          atomic_inc(&cpu_updated);
>>>      atomic_inc(&cpu_out);
>>>  
>>> +    if ( ret == -EIO )
>>> +    {
>>> +        printk(XENLOG_ERR
>>> +               "Late loading aborted: CPU%u failed to update ucode\n", 
>>> cpu);
>>> +        set_state(LOADING_EXIT);
>>> +        return ret;
>>> +    }
>>> +
>>> +    /* Let primary threads load the given ucode update */
>>> +    set_state(LOADING_ENTER);
>>
>> While the description goes to some lengths to explain this ordering of
>> updates, I still don't really see the point: How is it better for the
>> control CPU to have updated its ucode early and then hit an NMI before
>> the other CPUs have even started updating, than the other way around
>> in the opposite case?
> 
> We want to be conservative here. If an ucode is to update something
> shared by a whole socket, for the latter case, control thread may
> be accessing things that are being updating by the ucode loading on
> other cores. It is not safe, just like sibling thread isn't expected
> to access features exposed by the old ucode when primary thread is
> loading ucode.

Ah yes, considering a socket-wide effect didn't occur to me (although
it should have). So if you mention this aspect in the description, I
think I'm going to be fine with the change in this regard. Yet (as so
often) this raises another question: What about "secondary" sockets?
Shouldn't we entertain a similar two-step approach there then?

Jan

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