Re: [PATCH v19 027/130] KVM: TDX: Define TDX architectural definitions

Huang, Kai
2024-04-16T22:06:18Z

On 17/04/2024 4:28 am, Yamahata, Isaku wrote:
> On Tue, Apr 16, 2024 at 12:55:33PM +1200,
> "Huang, Kai" <kai.huang@intel.com> wrote:
>
>>
>>
>> On 5/03/2024 9:21 pm, Isaku Yamahata wrote:
>>> On Fri, Mar 01, 2024 at 03:25:31PM +0800,
>>> Yan Zhao <yan.y.zhao@intel.com> wrote:
>>>
>>>>> + * TD_PARAMS is provided as an input to TDH_MNG_INIT, the size of which is 1024B.
>>>>> + */
>>>>> +#define TDX_MAX_VCPUS (~(u16)0)
>>>> This value will be treated as -1 in tdx_vm_init(),
>>>> "kvm->max_vcpus = min(kvm->max_vcpus, TDX_MAX_VCPUS);"
>>>>
>>>> This will lead to kvm->max_vcpus being -1 by default.
>>>> Is this by design or just an error?
>>>> If it's by design, why not set kvm->max_vcpus = -1 in tdx_vm_init() directly.
>>>> If an unexpected error, may below is better?
>>>>
>>>> #define TDX_MAX_VCPUS (int)((u16)(~0UL))
>>>> or
>>>> #define TDX_MAX_VCPUS 65536
>>>
>>> You're right. I'll use ((int)U16_MAX).
>>> As TDX 1.5 introduced metadata MAX_VCPUS_PER_TD, I'll update to get the value
>>> and trim it further. Something following.
>>>
>>
>> [...]
>>
>>> + u16 max_vcpus_per_td;
>>> +
>>
>> [...]
>>
>>> - kvm->max_vcpus = min(kvm->max_vcpus, TDX_MAX_VCPUS);
>>> + kvm->max_vcpus = min3(kvm->max_vcpus, tdx_info->max_vcpus_per_td,
>>> + TDX_MAX_VCPUS);
>>
>> [...]
>>
>>> -#define TDX_MAX_VCPUS (~(u16)0)
>>> +#define TDX_MAX_VCPUS ((int)U16_MAX)
>>
>> Why do you even need TDX_MAX_VCPUS, given it cannot exceed U16_MAX and you
>> will have the 'u16 max_vcpus_per_td' anyway?
>>
>> IIUC, in KVM_ENABLE_CAP(KVM_CAP_MAX_VCPUS), we can overwrite the
>> kvm->max_vcpus to the 'max_vcpus' provided by the userspace, and make sure
>> it doesn't exceed tdx_info->max_vcpus_per_td.
>>
>> Anything I am missing?
>
> With the latest TDX 1.5 module, we don't need TDX_MAX_VCPUS.
>
> The metadata MD_FIELD_ID_MAX_VCPUS_PER_TD was introduced at the middle version
> of TDX 1.5. (I don't remember the exact version.), the logic was something
> like as follows. Now if we fail to read the metadata, disable TDX.
>
> read metadata MD_FIELD_ID_MAX_VCPUS_PER_TD;
> if success
> tdx_info->max_vcpu_per_td = the value read metadata
> else
> tdx_info->max_vcpu_per_td = TDX_MAX_VCPUS;
>

OK. But even the SEAMCALL can fail, we can just use U16_MAX directly
when it fails given we can see clearly the type of max_vcpu_per_td is 'u16'.

if success
tdx_info->max_vcpu_per_td = the value read metadata
else
tdx_info->max_vcpu_per_td = U16_MAX;

So I don't see why TDX_MAX_VCPUS is needed (especially in tdx_arch.h as
it is not an architectural value but just some value chosen for our
convenience).