Re: [Xen-devel] [PATCH v2 3/7] x86: detect and initialize Intel CAT feature

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.03.15 at 13:44, <dario.faggioli@xxxxxxxxxx> wrote:
> On Thu, 2015-03-19 at 18:41 +0800, Chao Peng wrote:
> 
>> diff --git a/docs/misc/xen-command-line.markdown 
> b/docs/misc/xen-command-line.markdown
>> index 63871cb..768c55f 100644
>> --- a/docs/misc/xen-command-line.markdown
>> +++ b/docs/misc/xen-command-line.markdown
>> @@ -1090,9 +1090,9 @@ This option can be specified more than once (up to 8 
> times at present).
>>  > `= <integer>`
>>  
>>  ### psr (Intel)
>> -> `= List of ( cmt:<boolean> | rmid_max:<integer> )`
>> +> `= List of ( cmt:<boolean> | rmid_max:<integer> | cat:<boolean> | 
> num_sockets:<interger> )`
>>
> As Tim, I also don't like the 'num_sockets' part of the interface.
> 
> Actually, I'm not sure I understand what the idea behind this parameter
> is. You say:
>  
>> +* Cache Alllocation Technology (Broadwell and later).  Information regarding
>> +  the cache allocation.
>> +  * `cat` instructs Xen to enable/disable Cache Allocation Technology.
>> +  * `num_sockets` indicates the number of available sockets for CAT feature
>> +    detection. All the sockets up to num_sockets will be checked for CAT
>> +    feature. 
>>
> What it the use case of specifying, say, num_sockets=2, on a 4 socket
> system? 
> 
> Is it for making it possible for the user to disable the feature on some
> of the socket? If yes, I think it does make sense, but then why always
> the last ones? shouldn't this be a list of socket  IDs?

I had asked about this before, and the answer was that this is to
_extend_ the range of sockets, to cover the CPU hotplug case. But
of course I could see ways to at least default the count to something
reasonable even taking hotplug into account (like taking the boot
time CPUs-per-socket value and dividing nr_cpu_ids by that value).

Jan


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