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

[Peter Fang]

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 to several megabytes.

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 AI tools (Claude:claude-opus-4-7, GitHub Copilot:gpt-5.4) 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.

[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 | 52 ++++++++++++++++++-------
 4 files changed, 60 insertions(+), 14 deletions(-)


base-commit: 4549871118cf616eecdd2d939f78e3b9e1dddc48
-- 
2.53.0