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

[Huang, Kai]

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.