Re: [Xen-devel] [PATCH v4 07/15] x86: implement set value flow for MBA

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Oct 05, 2017 at 04:48:12AM +0000, Yi Sun wrote:
> On 17-10-03 23:59:46, Jan Beulich wrote:
> > >>> Yi Sun <yi.y.sun@xxxxxxxxxxxxxxx> 09/29/17 4:58 AM >>>
> > >On 17-09-28 05:36:11, Jan Beulich wrote:
> > >> >>> On 23.09.17 at 11:48, <yi.y.sun@xxxxxxxxxxxxxxx> wrote:
> > >> > -            feat->cos_reg_val[cos * cos_num + i] = info->val[i];
> > >> > -            props->write_msr(cos, info->val[i], props->type[i]);
> > >> > +            if ( feat->cos_reg_val[cos * cos_num + j] != 
> > >> > val_array[index + j] )
> > >> > +                feat->cos_reg_val[cos * cos_num + j] =
> > >> > +                    props->write_msr(cos, val_array[index + j], 
> > >> > props->type[j]);
> > >> 
> > >> This renders partly useless the check: If hardware can alter the
> > >> value, repeatedly requesting the same value to be written will
> > >> no longer guarantee the MSR write to be skipped. If hardware
> > >> behavior can't be predicted you may want to consider recording
> > >> both the value in found by reading back the register written and
> > >> the value that was written - a match with either would eliminate
> > >> the need to do the write.
> > >> 
> > >The hardware behavior is explicitly defined by SDM and mentioned in
> > >'xl-psr.markdown' and 'intel_psr_mba.pandoc'. User should know that HW
> > >can alter MBA value if the value is not valid.
> > 
> > So if hardware behavior is fully defined, why don't you pre-adjust what is
> > to be written to the value hardware would alter it to?
> > 
> In previous version of MBA patch set, I pre-adjust the value in 
> 'mba_check_thrtl'.
> But Roger did not like that. So, the pre-adjust codes are removed.

IMHO it's quite pointless to do such adjustments when the hardware
performs them already. Also, I fear that our adjustments might get
out-of-sync in the future with what hardware actually does.

Maybe the result read back from the hardware (ie: adjusted) can be
stored and used in order to check whether a new value should be
written or not when switching? (I think this is the same that Jan
suggested above).

Roger.

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