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

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

On 17-10-05 03:39:06, Jan Beulich wrote:
> >>> On 05.10.17 at 10:39, <roger.pau@xxxxxxxxxx> wrote:
> > 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).
> 
> Not exactly, no - I'd like to avoid the write for _any_ value
> resulting in the one currently stored in the hardware register.
> Hence my earlier question on whether the transformation
> done by hardware is well defined (i.e. _not_ model dependent
> or fully defined by CPUID output).
> 
SDM does not mention it is model dependent. So, it is _not_ model dependent.

> Jan

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