[PATCH v2 0/2] tdx-guest: Make Quote buffer size dynamic

[Peter Fang <peter.fang@intel.com>]

Hi,

This series changes the TDX attestation driver's Quote buffer size from
a fixed constant to a value queried from the TDX module. So effectively:

  s/FIXED_BUF_SIZE/queried_buf_size/g

...in the TDX guest driver.

Terminology
===========

A "TD Quote" is an attestation structure signed with a platform key. It
contains information about a TDX guest and the platform it's running on.

The "Quote buffer" in the TDX guest driver is a memory buffer shared
between the TDX guest and the host VMM to retrieve TD Quotes. It has a
header defined in the GHCI spec [1].

Device Identifier Composition Engine ("DICE") provides a framework for
layering attestation evidence. This replaces the SGX model of contacting
an Intel server to obtain a certificate.

Problem
=======

The fixed-size Quote buffer approach is not sustainable. As
cryptographic algorithms evolve, TD Quote sizes also grow. A previous
commit [2] increased the guest driver's fixed-size Quote buffer to 128
KB to accommodate DICE Quotes, but it may still be insufficient when
those Quotes use post-quantum cryptography (PQC). PQC certificate chains
are roughly 10x-15x larger than conventional ones, which can increase
Quote sizes significantly.

What's in this series
=====================

To avoid changing the driver whenever the Quote buffer becomes too
small, newer TDX modules report their maximum Quote size via a metadata
field. The guest driver uses this value for its Quote buffer when
available. Older TDX modules continue to use the 128 KB buffer.

The changes do not affect configfs-tsm-report ABIs.

Patch 1/2: Add a helper to read the QUOTE_MAX_SIZE metadata field.
Patch 2/2: Replace the fixed Quote buffer size with the queried value,
           when available.

AI use
======

I used Claude:claude-opus-4-8 to proofread this cover letter and the
changelogs. The series also underwent AI code review (Claude:claude-opus-4-7),
but the feedback was limited to style suggestions.

v1: https://lore.kernel.org/all/20260612110853.3188196-1-peter.fang@intel.com/

Changes in v2:
 - Collect Reviewed-by tags. [Kiryl, Xiaoyao, Binbin, Sathya]
 - Keep the explicit (u32) cast in tdx_get_max_quote_size(). [Binbin]
 - Calculate the Quote buffer size with struct_size_t(). [Kiryl, Binbin]
 - Add __GFP_NOWARN to the allocation since its size comes from the
   host. [sashiko]
 - Rename quote_data_size to quote_data_len. [Sathya]
 - Drop the Assisted-by tags, as AI was not used to write the code.

[1] Guest Hypervisor Communication Interface (GHCI) Specification,
    Version 1.5, Section "TDG.VP.VMCALL<GetQuote>"
[2] 43185067c6fd ("configfs-tsm-report: tdx_guest: Increase Quote buffer
    size to 128KB")

Kuppuswamy Sathyanarayanan (1):
  virt: tdx-guest: Allocate Quote buffer dynamically

Peter Fang (1):
  x86/tdx: Add helper to query maximum TD Quote size

 arch/x86/coco/tdx/tdx.c                 | 19 +++++++++
 arch/x86/include/asm/shared/tdx.h       |  1 +
 arch/x86/include/asm/tdx.h              |  2 +
 drivers/virt/coco/tdx-guest/tdx-guest.c | 55 ++++++++++++++++++-------
 4 files changed, 62 insertions(+), 15 deletions(-)


base-commit: a13c140cc289c0b7b3770bce5b3ad42ab35074aa
-- 
2.53.0