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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Apr 22, 2021 at 01:42:45PM +0200, Jan Beulich wrote:
> On 22.04.2021 13:37, Roger Pau Monné wrote:
> > 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.
> 
> But we're not talking about actual MSR values here, as this is all
> about policy. So in the end we'll have to see how things need to
> be once we have the first non-feature-flag-like entries there. It
> feels as if simply zeroing can't be generally the right thing in
> such a case. It may e.g. be that min() is wanted instead.

Maybe level_msr should return an error for MSRs not explicitly
handled, that's propagated to the caller of
xc_cpu_policy_calc_compatible.

That way addition of new MSRs are not likely to miss adding the
required handling in level_msr?

Thanks, Roger.