Re: [PATCH v5 5/6] x86/tdx: Add a helper to query maximum Quote size

[Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>]

Hi,

On 9/28/2026 3:08 AM, Peter Fang wrote:
> TDX attestation report ("Quote") sizes can grow with newer crypto
> algorithms, so guests can no longer rely on a fixed-size buffer for the
> Quote.
> 
> The TDX module added a new ABI that reports the largest possible Quote
> size via a metadata field [1]. Add a helper to query the size instead of
> exposing tdg_vm_rd() directly, as it can read arbitrary metadata fields.
> 
> The reported size covers every Quote type the platform can produce,
> including SGX-based Quotes.
> 
> Thanks to Xu Yilun for suggesting this in an off-list discussion.

If the patch is suggested by Xu Uilun you can use Suggested-by:

> 
> AI was used under supervision to collect/apply feedback, review code and
> workshop logs.
> 
> [1] Intel TDX Module ABI Definitions, August 2026, TD-scope metadata
>     field "TD_QUOTE_MAX_SIZE"
> 
> Signed-off-by: Peter Fang <peter.fang@intel.com>
> ---
> v5:
>  - Drop unused EXPORT_SYMBOL_GPL(). [Dave]
>  - Use an error code to report failure. [Dave]
>  - Drop the u32 cast. [Dave]
>  - Replace the kernel-doc comment with a one-liner. [Dave]
>  - Drop the RB tags, as the code changed substantially.
> v4:
>  - Update the TDCS_QUOTE_MAX_SIZE encoding to 0x9010000200000007. [1]
>  - Provide documentation for the metadata field. [Rick, Kiryl]
>  - Document that the reported size covers every Quote type. [Xiaoyao]
>  - Document that a module update does not change the reported size.
>    [Tony]
>  - Add Tony's Reviewed-by.
> v3:
>  - No code changes. Add Binbin's Reviewed-by.
> v2:
>  - Keep the explicit (u32) cast to document the metadata field width.
>    [Binbin]
>  - Note that Xu Yilun's suggestion was made in an off-list discussion.
>    [Sathya]
>  - Drop the Assisted-by tags, as the code was not written by AI.
> ---
>  arch/x86/coco/tdx/tdx.c           | 16 ++++++++++++++++
>  arch/x86/include/asm/shared/tdx.h |  1 +
>  arch/x86/include/asm/tdx.h        |  2 ++
>  3 files changed, 19 insertions(+)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index bcaf4180a8db..323e1efc78ea 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -198,6 +198,22 @@ u64 tdx_hcall_get_quote(void *buf, size_t size)
>  }
>  EXPORT_SYMBOL_GPL(tdx_hcall_get_quote);
>  
> +/*
> + * Ask the TDX module what the largest possible Quote might be.
> + */
> +int tdx_get_max_quote_size(u64 *max_quote_size)
> +{
> +	u64 err;
> +
> +	err = tdg_vm_rd(TDCS_QUOTE_MAX_SIZE, max_quote_size);
> +
> +	/* Old modules do not support this. Tell the caller. */
> +	if (err)
> +		return -EINVAL;

I think -ENODEV or -EOPNOTSUPP is more appropriate return value
for unsupported case.

Also tdg_vm_rd() updates max_quote_size on error case as well.

You can reset it or use local variable to skip passing incorrect
value on error case.

> +
> +	return 0;
> +}
> +
>  static void __noreturn tdx_panic(const char *msg)
>  {
>  	struct tdx_module_args args = {
> diff --git a/arch/x86/include/asm/shared/tdx.h b/arch/x86/include/asm/shared/tdx.h
> index f20e91d7ac35..6f106d4b1a58 100644
> --- a/arch/x86/include/asm/shared/tdx.h
> +++ b/arch/x86/include/asm/shared/tdx.h
> @@ -50,6 +50,7 @@
>  /* TDX TD-Scope Metadata. To be used by TDG.VM.WR and TDG.VM.RD */
>  #define TDCS_CONFIG_FLAGS		0x1110000300000016
>  #define TDCS_TD_CTLS			0x1110000300000017
> +#define TDCS_QUOTE_MAX_SIZE		0x9010000200000007
>  #define TDCS_NOTIFY_ENABLES		0x9100000000000010
>  #define TDCS_TOPOLOGY_ENUM_CONFIGURED	0x9100000000000019
>  
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index a3c37d61e676..6a465827d8f9 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -83,6 +83,8 @@ int tdx_mcall_extend_rtmr(u8 index, u8 *data);
>  
>  u64 tdx_hcall_get_quote(void *buf, size_t size);
>  
> +int tdx_get_max_quote_size(u64 *max_quote_size);
> +
>  void __init tdx_dump_attributes(u64 td_attr);
>  void __init tdx_dump_td_ctls(u64 td_ctls);
>  

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer