Re: [Xen-devel] [PATCH v12 13/23] x86: refactor psr: CDP: implement CPU init flow.

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx> 06/30/17 10:05 AM >>>
>On 17-06-30 01:33:02, Jan Beulich wrote:
>> >>> Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx> 06/30/17 9:01 AM >>>
>> >This accords to spec:
>> >"For CDP operations, COS_MAX_CDP is equal to (CPUID.(EAX=10H, 
>> >ECX=1):EDX.COS_MAX_CAT >>1)."
>> >
>> >HW should make sure it is even number.
>> 
>> And how about someone using the command line option to shrink the to be used 
>> set?
>> 
>Good question. The command line option saved in 'opt_cos_max', even it is
>not even number and less than 'EDX.COS_MAX_CAT' so that the 'cos_max' here is
>same as it, the right shift operation does not cause any issue I think.
>
>The description in docs/misc/xen-command-line.markdown is clear that "the 
>cos_max
>in use will automatically reduce to half when CDP is enabled". E.g. 
>'opt_cos_max'
>is 5, then we get 2 for CDP. I think user should be aware of this by reading 
>the
>markdown file.

I don't really follow all this argumentation - what's the problem of simply 
subtracting
1 before doing the shift? Talking of which - isn't the general cos_max > 1 also
insufficient for the CDP case?

Jan


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