Re: [PATCH v10 05/25] x86: refactor psr: L3 CAT: implement CPU init and free flow.

[Yi Sun <yi.y.sun@linux.intel.com>]

On 17-04-06 08:02:15, Jan Beulich wrote:
> >>> On 06.04.17 at 12:02, <yi.y.sun@linux.intel.com> wrote:
> > On 17-04-06 03:34:27, Jan Beulich wrote:
> >> >>> On 06.04.17 at 11:22, <yi.y.sun@linux.intel.com> wrote:
> >> > On 17-04-06 02:32:04, Jan Beulich wrote:
> >> >> >>> On 06.04.17 at 07:49, <yi.y.sun@linux.intel.com> wrote:
> >> >> > On 17-04-05 09:10:58, Jan Beulich wrote:
> >> >> >> >>> On 01.04.17 at 15:53, <yi.y.sun@linux.intel.com> wrote:
> >> >> >> > +static void cat_init_feature(const struct cpuid_leaf *regs,
> >> >> >> > +                             struct feat_node *feat,
> >> >> >> > +                             struct psr_socket_info *info,
> >> >> >> > +                             enum psr_feat_type type)
> >> >> >> > +{
> >> >> >> > +    unsigned int socket, i;
> >> >> >> > +
> >> >> >> > +    /* No valid value so do not enable feature. */
> >> >> >> > +    if ( !regs->a || !regs->d )
> >> >> >> > +        return;
> >> >> >> > +
> >> >> >> > +    feat->props->cbm_len = (regs->a & CAT_CBM_LEN_MASK) + 1;
> >> >> >> > +    feat->props->cos_max = min(opt_cos_max, regs->d & CAT_COS_MAX_MASK);
> >> >> >> > +
> >> >> >> > +    switch ( type )
> >> >> >> > +    {
> >> >> >> > +    case PSR_SOCKET_L3_CAT:
> >> >> >> > +        /* cos=0 is reserved as default cbm(all bits within cbm_len are 1). */
> >> >> >> > +        feat->cos_reg_val[0] = cat_default_val(feat->props->cbm_len);
> >> >> >> > +
> >> >> >> > +        /*
> >> >> >> > +         * To handle cpu offline and then online case, we need restore MSRs to
> >> >> >> > +         * default values.
> >> >> >> > +         */
> >> >> >> > +        for ( i = 1; i <= feat->props->cos_max; i++ )
> >> >> >> > +        {
> >> >> >> > +            wrmsrl(MSR_IA32_PSR_L3_MASK(i), feat->cos_reg_val[0]);
> >> >> >> > +            feat->cos_reg_val[i] = feat->cos_reg_val[0];
> >> >> >> > +        }
> >> >> >> 
> >> >> >> I continue to have difficulty with this: Why is offline-then-online
> >> >> >> any different from first-time-online? Why wouldn't setting the
> >> >> > 
> >> >> > May remove this comment. Per current codes, the MSRs are written to default
> >> >> > values no matter first time or not.
> >> >> > 
> >> >> >> registers to their intended values not be taken care of by
> >> >> >> context switch code, once vCPU-s get scheduled onto the newly
> >> >> >> onlined CPU?
> >> >> >> 
> >> >> > cat_init_feature is only called when the first CPU on a socket is online.
> >> >> > The MSRs to set are per socket. So, we only need set it once when socket
> >> >> > is online.
> >> >> 
> >> >> This does not answer my question. Once again - why does this need
> >> >> doing here explicitly, rather than relying on the needed values being
> >> >> loaded when the first vCPU gets scheduled onto one of the pCPU-s
> >> >> of this socket?
> >> >> 
> >> > I do not know if I understand your question correctly. Let me try to explain
> >> > again. As we discussed in v9, the MSRs values may be wrong values when socket
> >> > is online. That is the reason we have to restore them. 
> >> > 
> >> > The MSRs are per socket. That means only one group of MSRs on one socket. So
> >> > the setting on one CPU can make it valid on whole socket. The 'cat_init_feature'
> >> > is executed when the first CPU on a socket is online so we restore them here.
> >> 
> >> All understood. But you write the MSRs with the needed values in
> >> the context switch path, don't you? Why is that writing not
> >> sufficient?
> >> 
> > No, in context switch path, we only set ASSOC register to set COS ID into it so
> > that the corresponding COS MSR value (CBM) can work.
> 
> Okay, so not the context switch path then, But you must be
> changing the MSRs _somewhere_, and the question is why this
> somewhere isn't sufficient.
> 
Besides the restore behavior in init process, I restore the MSRs when ref[cos]
is reduced to 0. This behavior happens in two scenarios:
1. In a value setting process, restore MSR if the ref[old_cos] is reduced to 0.
2. When a domain is destroyed, its ref[cos] is reduced too.

Reason to restore is below:
  For features,  e.g. CDP, which cos_num is more than 1, we have to
  restore the old_cos value back to default when ref[old_cos] is 0.
  Otherwise, user will see wrong values when this COS ID is reused. E.g.
  user wants to set DATA to 0x3ff for a new domain. He hopes to see the
  DATA is set to 0x3ff and CODE should be the default value, 0x7ff. But
  if the COS ID picked for this action is the one that has been used by
  other domain and the CODE has been set to 0x1ff. Then, user will see
  DATA: 0x3ff, CODE: 0x1ff. So, we have to restore COS values for features
  using multiple COSs.

BRs,
Sun Yi

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel