Re: [PATCH 4/5] x86/virt/tdx: Support global metadata read for all element sizes

[Huang, Kai]

On 3/05/2024 12:19 pm, Edgecombe, Rick P wrote:
> On Sat, 2024-03-02 at 00:20 +1300, Kai Huang wrote:
> > For now the kernel only reads TDMR related global metadata fields for
> > module initialization.  All these fields are 16-bits, and the kernel
> > only supports reading 16-bits fields.
> >
> > KVM will need to read a bunch of non-TDMR related metadata to create and
> > run TDX guests.  It's essential to provide a generic metadata read
> > infrastructure which supports reading all 8/16/32/64 bits element sizes.
> >
> > Extend the metadata read to support reading all these element sizes.
>
> It makes it sound like KVM needs 8 bit fields. I think it only needs 16 and 64.
> (need to verify fully) But the code to support those can be smaller if it's
> generic to all sizes.
>
> It might be worth mentioning which fields and why to make it generic. To make
> sure it doesn't come off as a premature implementation.

How about:

For now the kernel only reads TDMR related global metadata fields for
module initialization.  All these fields are 16-bits, and the kernel
only supports reading 16-bits fields.

KVM will need to read a bunch of non-TDMR related metadata to create and
run TDX guests, and KVM will at least need to additionally be able to 
read 64-bit metadata fields.

For example, the KVM will need to read the 64-bit ATTRIBUTES_FIXED{0|1} 
fields to determine whether a particular attribute is legal when 
creating a TDX guest.  Please see 'global_metadata.json in [1] for more 
information.

To resolve this once for all, extend the existing metadata reading code 
to provide a generic metadata read infrastructure which supports reading 
all 8/16/32/64 bits element sizes.

[1] https://cdrdv2.intel.com/v1/dl/getContent/795381