Re: [PATCH] x86/cpu-policy: Extend the guest max policy max leaf/subleaves

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

On Wed, Oct 30, 2024 at 04:51:34PM +0000, Andrew Cooper wrote:
> On 30/10/2024 3:13 pm, Roger Pau Monné wrote:
> > On Wed, Oct 30, 2024 at 02:45:19PM +0000, Andrew Cooper wrote:
> >> On 30/10/2024 11:03 am, Roger Pau Monné wrote:
> >>> On Wed, Oct 30, 2024 at 10:39:12AM +0000, Andrew Cooper wrote:
> >>>> On 30/10/2024 8:59 am, Roger Pau Monné wrote:
> >>>>> On Tue, Oct 29, 2024 at 05:55:05PM +0000, Andrew Cooper wrote:
> >>>>>> diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
> >>>>>> index b6d9fad56773..78bc9872b09a 100644
> >>>>>> --- a/xen/arch/x86/cpu-policy.c
> >>>>>> +++ b/xen/arch/x86/cpu-policy.c
> >>>>>> @@ -391,6 +391,27 @@ static void __init calculate_host_policy(void)
> >>>>>>      p->platform_info.cpuid_faulting = cpu_has_cpuid_faulting;
> >>>>>>  }
> >>>>>>  
> >>>>>> +/*
> >>>>>> + * Guest max policies can have any max leaf/subleaf within bounds.
> >>>>>> + *
> >>>>>> + * - Some incoming VMs have a larger-than-necessary feat max_subleaf.
> >>>>>> + * - Some VMs we'd like to synthesise leaves not present on the host.
> >>>>>> + */
> >>>>>> +static void __init guest_common_max_leaves(struct cpu_policy *p)
> >>>>>> +{
> >>>>>> +    p->basic.max_leaf       = ARRAY_SIZE(p->basic.raw) - 1;
> >>>>>> +    p->feat.max_subleaf     = ARRAY_SIZE(p->feat.raw) - 1;
> >>>>>> +    p->extd.max_leaf        = 0x80000000U + ARRAY_SIZE(p->extd.raw) - 
> >>>>>> 1;
> >>>>>> +}
> >>>>>> +
> >>>>>> +/* Guest default policies inherit the host max leaf/subleaf settings. 
> >>>>>> */
> >>>>>> +static void __init guest_common_default_leaves(struct cpu_policy *p)
> >>>>>> +{
> >>>>>> +    p->basic.max_leaf       = host_cpu_policy.basic.max_leaf;
> >>>>>> +    p->feat.max_subleaf     = host_cpu_policy.feat.max_subleaf;
> >>>>>> +    p->extd.max_leaf        = host_cpu_policy.extd.max_leaf;
> >>>>>> +}
> >>>>> I think this what I'm going to ask is future work.  After the
> >>>>> modifications done to the host policy by max functions
> >>>>> (calculate_{hvm,pv}_max_policy()) won't the max {sub,}leaf adjustments
> >>>>> better be done taking into account the contents of the policy, rather
> >>>>> than capping to the host values?
> >>>>>
> >>>>> (note this comment is strictly for guest_common_default_leaves(), the
> >>>>> max version is fine using ARRAY_SIZE).
> >>>> I'm afraid I don't follow.
> >>>>
> >>>> calculate_{pv,hvm}_max_policy() don't modify the host policy.
> >>> Hm, I don't think I've expressed myself clearly, sorry.  Let me try
> >>> again.
> >>>
> >>> calculate_{hvm,pv}_max_policy() extends the host policy by possibly
> >>> setting new features, and such extended policy is then used as the
> >>> base for the PV/HVM default policies.
> >>>
> >>> Won't the resulting policy in calculate_{hvm,pv}_def_policy() risks
> >>> having bits set past the max {sub,}leaf in the host policy, as it's
> >>> based in {hvm,pv}_def_cpu_policy that might have such bits set?
> >> Oh, right.
> >>
> >> This patch doesn't change anything WRT that.
> > Indeed, didn't intend my comment to block it, just that I think at
> > some point the logic in guest_common_default_leaves() will need to be
> > expanded.
> >
> >> But I think you're right that we do risk getting into that case (in
> >> principle at least) because of how guest_common_*_feature_adjustment() 
> >> work.
> >>
> >> Furthermore, the bug will typically get hidden because we serialise
> >> based on the max_leaf/subleaf, and will discard feature words outside of
> >> the max_leaf/subleaf bounds.
> > Yes, once we serialize it for toolstack consumption the leafs will be
> > implicitly zeroed.
> >
> >> I suppose we probably want a variation of x86_cpu_featureset_to_policy()
> >> which extends the max_leaf/subleaf based on non-zero values in leaves. 
> >> (This already feels like it's going to be an ugly algorithm.)
> > Hm, I was thinking that we would need to adjust
> > guest_common_default_leaves() to properly shrink the max {sub,}leaf
> > fields from the max policies.
> 
> Hmm.  What we'd do is have default inherit max's ARRAY_SIZES(), then do
> all the existing logic, then as the final step, shrink the default
> policies, vaguely per Jan's plan.
> 
> i.e. we'd end up deleting guest_common_default_leaves()
> 
> That way we don't need to encode any knowledge of which feature bit
> means what WRT max_leaf/subleaf.

What about Alejandro's concern about runtime populated {sub,}leafs,
won't we risk shrinking too much if the last leaf intended to be kept
happens to be a fully runtime populated one?

Do we need some kind of special magic for fully run-time populated
leafs (like the topology ones IIRC?) in order to account for them when
doing those max calculations?

Thanks, Roger.