Re: [PATCH v2 15/21] libs/guest: obtain a compatible cpu policy from two input ones

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

On Thu, Apr 22, 2021 at 01:05:23PM +0200, Jan Beulich wrote:
> On 22.04.2021 12:56, Roger Pau Monné wrote:
> > On Thu, Apr 22, 2021 at 12:48:36PM +0200, Jan Beulich wrote:
> >> On 22.04.2021 12:34, Roger Pau Monné wrote:
> >>> On Thu, Apr 22, 2021 at 11:58:45AM +0200, Jan Beulich wrote:
> >>>> On 22.04.2021 11:42, Roger Pau Monné wrote:
> >>>>> On Wed, Apr 14, 2021 at 03:49:02PM +0200, Jan Beulich wrote:
> >>>>>> On 13.04.2021 16:01, Roger Pau Monne wrote:
> >>>>>>> @@ -944,3 +945,130 @@ bool xc_cpu_policy_is_compatible(xc_interface 
> >>>>>>> *xch, const xc_cpu_policy_t host,
> >>>>>>>  
> >>>>>>>      return false;
> >>>>>>>  }
> >>>>>>> +
> >>>>>>> +static uint64_t level_msr(unsigned int index, uint64_t val1, 
> >>>>>>> uint64_t val2)
> >>>>>>> +{
> >>>>>>> +    uint64_t val = val1 & val2;;
> >>>>>>
> >>>>>> For arbitrary MSRs this isn't going to do any good. If only very
> >>>>>> specific MSRs are assumed to make it here, I think this wants
> >>>>>> commenting on.
> >>>>>
> >>>>> I've added: "MSRs passed to level_msr are expected to be bitmaps of
> >>>>> features"
> >>>>
> >>>> How does such a comment help? I.e. how does the caller tell which MSRs
> >>>> to pass here and which to deal with anyother way?
> >>>
> >>> All MSRs should be passed to level_msr, but it's handling logic would
> >>> need to be expanded to support MSRs that are not feature bitmaps.
> >>>
> >>> It might be best to restore the previous switch and handle each MSR
> >>> specifically?
> >>
> >> I think so, yes. We need to be very careful with what a possible
> >> default case does there, though.
> > 
> > Maybe it would be better to handle level_msr in a way similar to
> > level_leaf: return true/false to notice whether the MSR should be
> > added to the resulting compatible policy?
> > 
> > And then make the default case in level_msr just return false in order
> > to prevent adding MSRs not properly leveled to the policy?
> 
> I'm afraid I'm not clear about the implications. What again is the
> (planned?) final effect of an MSR not getting added there?

Adding the MSR with a 0 value will zero out any previous value on the
'out' policy, while not adding it would leave the previous value
there given the current code in xc_cpu_policy_calc_compatible added by
this patch.

I would expect callers of xc_cpu_policy_calc_compatible to pass a
zeroed 'out' policy, so I think the end result should be the same.

Maybe I should also clear 'out' in xc_cpu_policy_calc_compatible, at
which point both options will get the same exact result.

Thanks, Roger.