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

[Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx>]

On 17-03-13 06:37:41, Jan Beulich wrote:
> >>> 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.
> 
Considering to support future feature, we use 64bit as input parameter type for
psr_get_val/psr_set_val. So, we handle all values as 64bit.

For psr_check_cbm case, I think we can add an ASSERT as below. Is that good for
you? Thanks!

ASSERT(cbm_len <= 32);

> Jan
> 
> 
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