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 05:39:07 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "Li, Xiaoyao" <xiaoyao.li@...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 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.
"

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