Re: [PATCH v2 02/10] x86/virt/tdx: Unbind global metadata read with 'struct tdx_tdmr_sysinfo'

[Dan Williams <dan.j.williams@intel.com>]

Kai Huang wrote:
> The TDX module provides a set of "global metadata fields".  They report
> things like TDX module version, supported features, and fields related
> to create/run TDX guests and so on.
> 
> For now the kernel only reads "TD Memory Region" (TDMR) related global
> metadata fields to a 'struct tdx_tdmr_sysinfo' for initializing the TDX
> module, and the metadata reading code can only work with that structure.
> 
> Future changes will need to read other metadata fields that don't make
> sense to populate to the "struct tdx_tdmr_sysinfo".  It's essential to
> provide a generic metadata read infrastructure which is not bound to any
> specific structure.
> 
> To start providing such infrastructure, unbind the metadata reading code
> with the 'struct tdx_tdmr_sysinfo'.
> 
> Note the kernel has a helper macro, TD_SYSINFO_MAP(), for marshaling the
> metadata into the 'struct tdx_tdmr_sysinfo', and currently the macro
> hardcodes the structure name.  As part of unbinding the metadata reading
> code with 'struct tdx_tdmr_sysinfo', it is extended to accept different
> structures.
> 
> Unfortunately, this will result in the usage of TD_SYSINFO_MAP() for
> populating 'struct tdx_tdmr_sysinfo' to be changed to use the structure
> name explicitly for each structure member and make the code longer.  Add
> a wrapper macro which hides the 'struct tdx_tdmr_sysinfo' internally to
> make the code shorter thus better readability.
> 
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
> ---
> 
> v1 -> v2:
>  - 'st_member' -> 'member'. (Nikolay)
> 
> ---
>  arch/x86/virt/vmx/tdx/tdx.c | 25 ++++++++++++++-----------
>  1 file changed, 14 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
> index d8fa9325bf5e..2ce03c3ea017 100644
> --- a/arch/x86/virt/vmx/tdx/tdx.c
> +++ b/arch/x86/virt/vmx/tdx/tdx.c
> @@ -272,9 +272,9 @@ static int read_sys_metadata_field(u64 field_id, u64 *data)
>  
>  static int read_sys_metadata_field16(u64 field_id,
>  				     int offset,
> -				     struct tdx_tdmr_sysinfo *ts)
> +				     void *stbuf)

The loss of all type-safety sticks out, and points to the fact that
@offset was awkward to pass in from the beginning. I would have expected
a calling convention like:

static int read_sys_metadata_field16(u64 field_id, u16 *val)

...and make the caller calculate the buffer in a type-safe way.

The problem with the current code is that it feels like it is planning
ahead for a dynamic metdata reading future, that is not coming, Instead
it could be leaning further into initializing all metadata once.

In other words what is the point of defining:

static const struct field_mapping fields[]

...only to throw away all type-safety and run it in a loop. Why not
unroll the loop, skip the array, and the runtime warning with something
like?

read_sys_metadata_field16(MD_FIELD_ID_MAX_TDMRS, &ts->max_tdmrs);
read_sys_metadata_field16(MD_FIELD_ID_MAX_RESERVED_PER_TDMR, &ts->max_reserved_per_tdmr);
...etc

The unrolled loop is the same amount of work as maintaining @fields.