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

Huang, Kai
2024-03-15T05:39:39Z

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.