Re: [PATCH v8 02/11] x86/virt/tdx: Allocate page bitmap for Dynamic PAMT

On Sat, 2026-07-25 at 00:33 +0000, [email protected] wrote:
> [Severity: Medium]
> Does assigning val to pamt_page_bitmap_entry_bits result in an unsafe
> truncation?
> 
> Since pamt_page_bitmap_entry_bits is declared as a u8 in
> struct tdx_sys_info_tdmr, and the metadata field ID 0x9100000100000013
> specifies a 16-bit value according to the TDX ABI (bits 33:32 are '01'),
> a value from the hardware might be silently truncated.
> 
> If a future TDX module increases the required bitmap entry size to 256 bits
> or more, wouldn't this truncation cause severe memory under-allocation for
> the physically contiguous PAMT memory?

Impressed that Sashiko is so knowledgeable about TDX.

The spec lists pamt_page_bitmap_entry_bits as byte sized so there will be no
truncation. But 33:32 could be 00 and still work. The docs say:
   All metadata read and write interface functions (e.g.,
   TDH.MNG.RD, TDH.MNG.WR, TDH.VP.RD, TDG.SYS.RDALL etc.)
   ignore this field when used as an input; they use an implicit value.

Hmm... Both will work, but 00 for 33:32 would arguably be more correct even if
it is ignored. Hate to spin another version for this tiny non-functional thing.
Hmm...