linux-kernel - Re: [PATCH v19 034/130] KVM: TDX: Get system-wide info about TDX module on initialization

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Date: Fri, 15 Mar 2024 13:50:38 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Yamahata, Isaku" <isaku.yamahata@...el.com>
Cc: "Zhang, Tina" <tina.zhang@...el.com>, "Yuan, Hang" <hang.yuan@...el.com>,
 "seanjc@...gle.com" <seanjc@...gle.com>, "Chen, Bo2" <chen.bo@...el.com>,
 "sagis@...gle.com" <sagis@...gle.com>,
 "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
 "Aktas, Erdem" <erdemaktas@...gle.com>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v19 034/130] KVM: TDX: Get system-wide info about TDX
 module on initialization

On 3/15/2024 1:39 PM, Huang, Kai wrote:
> On Fri, 2024-03-15 at 13:11 +0800, Li, Xiaoyao wrote:
>>> Here is what Isaku can do using the current API:
>>>
>>>   	u64 num_cpuid_config;
>>   >
>>>
>>>   	...
>>>
>>>   	tdx_sys_metadata_field_read(NUM_CPUID_CONFIG, &num_cpuid_config);
>>>
>>>   	tdx_info = kzalloc(calculate_tdx_info_size(num_cpuid_config), ...);
>>>
>>>   	tdx_info->num_cpuid_config = num_cpuid_config;
>>
>> Dosen't num_cpuid_config serve as temporary variable in some sense?
> 
> You need it, regardless whether it is u64 or u16.
> 
>>
>> For this case, it needs to be used for calculating the size of tdx_info.
>> So we have to have it. But it's not the common case.
>>
>> E.g., if we have another non-u64 field (e.g., field_x) in tdx_info, we
>> cannot to read it via
>>
>> 	tdx_sys_metadata_field_read(FIELD_X_ID, &tdx_info->field_x);
>>
>> we have to use a temporary u64 variable.
> 
> Let me repeat below in my _previous_ reply:
> 
> "
> One example that the current tdx_sys_metadata_field_read() doesn't quite fit is
> you have something like this:
> 
> 	struct {
> 		u16 whatever;
> 		...
> 	} st;
> 
> 	tdx_sys_metadata_field_read(FIELD_ID_WHATEVER, &st.whatever);
> 
> But for this use case you are not supposed to use tdx_sys_metadata_field_read(),
> but use tdx_sys_metadata_read() which has a mapping provided anyway.
> "

tdx_sys_metadata_read() is too complicated for just reading one field.

Caller needs to prepare a one-item size array of "struct 
tdx_metadata_field_mapping" and pass the correct offset.

> So sorry I am not seeing a real example from you.
>