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

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Message-ID: <e0713aaa-0537-403b-9abe-db267e21f0b7@intel.com>
Date: Wed, 17 Apr 2024 10:06:03 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: "Yamahata, Isaku" <isaku.yamahata@...el.com>
CC: Isaku Yamahata <isaku.yamahata@...ux.intel.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, Paolo Bonzini
	<pbonzini@...hat.com>, "Aktas, Erdem" <erdemaktas@...gle.com>, "Sean
 Christopherson" <seanjc@...gle.com>, Sagi Shahar <sagis@...gle.com>, "Chen,
 Bo2" <chen.bo@...el.com>, "Yuan, Hang" <hang.yuan@...el.com>, "Zhang, Tina"
	<tina.zhang@...el.com>, Sean Christopherson
	<sean.j.christopherson@...el.com>, "Li, Xiaoyao" <Xiaoyao.Li@...el.com>
Subject: Re: [PATCH v19 027/130] KVM: TDX: Define TDX architectural
 definitions



On 17/04/2024 4:28 am, Yamahata, Isaku wrote:
> On Tue, Apr 16, 2024 at 12:55:33PM +1200,
> "Huang, Kai" <kai.huang@...el.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@...el.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).