Re: [Xen-devel] [PATCH RESEND v5 01/24] docs: create L2 Cache Allocation Technology (CAT) feature document

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> > +# Testing
> > +
> > +L2 CAT uses same xl interfaces as L3 CAT/CDP. So, we can execute these
> > +commands to verify L2 CAT and L3 CAT/CDP on different HWs support them.
> > +
> > +For example:
> > +    root@:~$ xl psr-hwinfo --cat
> > +    Cache Allocation Technology (CAT): L2
> > +    Socket ID       : 0
> > +    Maximum COS     : 3
> > +    CBM length      : 8
> > +    Default CBM     : 0xff
> > +
> > +    root@:~$ xl psr-cat-cbm-set -l2 1 0x7f
> > +
> > +    root@:~$ xl psr-cat-show -l2 1
> > +    Socket ID       : 0
> > +    Default CBM     : 0xff
> > +       ID                     NAME             CBM
> > +        1                 ubuntu14            0x7f
> > +

I tried finding the Intel SDM (December 2016) what the format
of CBM is as the value '0x7f' does not really mean much to me.

Right above Figure 17.28 it says:

"Figure 17-27 also shows three examples of sets of Cache Capacity Bitmasks. For 
simplicity these are represented
as 8-bit vectors, though this may vary _depending on the implementation and how 
the mask is mapped to the avail-
able cache capacity._"


So in other words - not documented. 

Then later is says:

" Rather, this is a convenient manner to represent capacity,
overlap and isolation of cache space. For example, executing a POPCNT 
instruction (population count of set bits) on
the capacity bitmask can provide the fraction of cache space that a class of 
service can allocate into."

OK, so _can_ (but not _MUST_), so again implementation specific - and
can provide a fraction of cache space.

Which would imply that the values could be:

0x0F - half of L2
0x03 - quarter of L2

Is there some other documentation that explains this in more details?

If it is like I mentioned would it make sense to have 'xl' be capable
of dealing with string values? such as "three-quarters", "half", etc
and then set it? Or percentage and map that to the correct value?

Like:

xl psr-cat-cbm-set -l2 ubuntu14 50%

would be quite obvious instead of say 0x0F?

Thanks.

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