[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] x86/cpu-policy: Extend the guest max policy max leaf/subleaves
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. ~Andrew
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