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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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