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 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?

Thanks, Roger.