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

["Alejandro Vallejo" <alejandro.vallejo@xxxxxxxxx>]

On Wed Oct 30, 2024 at 3:13 PM GMT, 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.

That would be tricky in case we end up with subleafs that are strictly
populated at runtime. Xen would have no way of knowing whether that's meant to
be implemented or not. It seems safer to raise the max if we find a non-zero
leaves higher than the current max.

The algorithm is probably quite simple for static data, as it's merely
traversing the raw arrays and keeping track of the last non-zero leaf.

>
> Thanks, Roger.

Cheers,
Alejandro