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

[Yi Sun <yi.y.sun@linux.intel.com>]

On 17-06-30 03:18:53, Jan Beulich wrote:
> >>> Yi Sun <yi.y.sun@linux.intel.com> 06/30/17 10:05 AM >>>
> >On 17-06-30 01:33:02, Jan Beulich wrote:
> >> >>> Yi Sun <yi.y.sun@linux.intel.com> 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

There will be problem if we substract 1 before doing the shift for the even
number. E.g. the original cos_max is 2, (2-1)>>1 will be 0. That is not we
want. For the odd number, direct right shift is good too. E.g. the original
cos_max is 3, 3 >> 1 is 1 which is what we want.

> insufficient for the CDP case?

We do not need check if cos_max > 1 because the 'cos_max = 0' works for us.
That means only COS ID 0 can be used. You can see, we use "cos <= cos_max" to
find COS ID. Of course, the ID 0 saves the default value which cannot be
overwritten.

> 
> Jan

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