Re: [Xen-devel] [PATCH v8 09/24] x86: refactor psr: set value: assemble features value array.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.03.17 at 03:43, <yi.y.sun@xxxxxxxxxxxxxxx> wrote:
> On 17-03-10 02:15:20, Jan Beulich wrote:
>> >>> On 10.03.17 at 04:21, <yi.y.sun@xxxxxxxxxxxxxxx> wrote:
>> > On 17-03-08 09:54:04, Jan Beulich wrote:
>> >> >>> On 15.02.17 at 09:49, <yi.y.sun@xxxxxxxxxxxxxxx> wrote:
>> >> > @@ -207,6 +233,29 @@ static enum psr_feat_type 
> psr_cbm_type_to_feat_type(enum cbm_type type)
>> >> >      return feat_type;
>> >> >  }
>> >> >  
>> >> > +static bool psr_check_cbm(unsigned int cbm_len, uint64_t cbm)
>> >> > +{
>> >> > +    unsigned int first_bit, zero_bit;
>> >> > +
>> >> > +    /* Set bits should only in the range of [0, cbm_len). */
>> >> > +    if ( cbm & (~0ull << cbm_len) )
>> >> 
>> >> This will not do what you intend for cbm_len == 64.
>> >> 
>> > cbm_len is not 64. cbm_len means the CBM value length, how many bits. For 
>> > L3
>> > CAT, it may be 11 bits. For L2 CAT, it may be 8 bits.
>> 
>> And there is an _architectural_ guarantee for them to never
>> reach 64 bits?
>> 
> Per SDM, 'EAX[4:0] reports the length of the capacity bitmask length'. So, we
> get cbm_len in '*_init_feature' function as below.
> 
> #define CAT_CBM_LEN_MASK 0x1f
> cat.cbm_len = (regs.a & CAT_CBM_LEN_MASK) + 1;
> 
> So, the max cbm_len is '32'.

Great, yet in that case why do you use 64-bit calculations at all?
Avoiding the undefinedness when cbm_len == 32 can be
achieved without 64-bit arithmetic.

Jan


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