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 01:33:02, Jan Beulich wrote:
> >>> Yi Sun <yi.y.sun@linux.intel.com> 06/30/17 9:01 AM >>>
> >On 17-06-30 00:40:35, Jan Beulich wrote:
> >> >>> Yi Sun <yi.y.sun@linux.intel.com> 06/14/17 3:26 AM >>>
> >> > @@ -253,6 +271,26 @@ static void cat_init_feature(const struct cpuid_leaf *regs,
> >> >  
> >> >          break;
> >> >  
> >> > +    case PSR_SOCKET_L3_CDP:
> >> > +    {
> >> > +        uint64_t val;
> >> > +
> >> > +        /* Cut half of cos_max when CDP is enabled. */
> >> > +        feat->cos_max >>= 1;
> >> 
> >> I'm afraid this is off by one in the unusual but possible case of cos_max
> >> being an even number.
> >> 
> >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.

> Jan

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