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 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?

Thanks, Roger.