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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.10.2024 17:51, 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.

Yet, beyond what my present patch has, not below anything the tool stack
has asked for explicitly.

Jan