Re: [PATCH 3/5] x86: add domctl cmd to set/get CDP code/data CBM

[He Chen <he.chen@linux.intel.com>]

On Wed, Sep 02, 2015 at 12:59:07PM +0100, Andrew Cooper wrote:
> On 02/09/15 09:28, He Chen wrote:
> > CDP extends CAT and provides the capacity to control L3 code & data
> > cache. With CDP, one COS correspond to two CMBs(code & data). cbm_type
> > is added to support distinguish different CBM operation. Besides, new
> > domctl cmds are introdunced to support set/get CDP CBM. Some CAT
> > functions to operation CBMs are extended to support CDP.
> >
> > Signed-off-by: He Chen <he.chen@linux.intel.com>
> > ---
> >  xen/arch/x86/domctl.c       |  33 +++++++++-
> >  xen/arch/x86/psr.c          | 142 ++++++++++++++++++++++++++++++++------------
> >  xen/include/asm-x86/psr.h   |  12 +++-
> >  xen/include/public/domctl.h |   4 ++
> >  4 files changed, 150 insertions(+), 41 deletions(-)
> >
> > diff --git a/xen/arch/x86/psr.c b/xen/arch/x86/psr.c
> > index 26596dd..8e92d24 100644
> > --- a/xen/arch/x86/psr.c
> > +++ b/xen/arch/x86/psr.c
> > +static int pick_avail_cos(struct psr_cat_cbm *map, int cos_max, int old_cos)
> > +{
> > +    int cos;
> > +
> > +    /* If old cos is referred only by the domain, then use it. */
> > +    if ( map[old_cos].ref == 1 )
> > +        return old_cos;
> > +
> > +    /* Then we pick an unused one, never pick 0 */
> > +    for ( cos = 1; cos <= cos_max; cos++ )
> > +        if ( map[cos].ref == 0 )
> > +            return cos;
> > +
> > +    return -EOVERFLOW;
> 
> ENOENT surely, or use EOVERFLOW consistently.
> 

I am not sure I got your point here. pick_avail_cos is to get an unused
COS, if succeed, it returns a positive number which means COS, but
when fail, it should return a negative number to indicate an error.

As far as I know, ENOENT is 2 and EOVERFLOW is 75, if I return ENOENT
directly, the function which call pick_avail_cos could not tell the
value is a valid COS or an error number.

Would you mind explaining in more detail for me and thanks.