Re: [PATCH 1/2] x86/tdx: Add helper to query maximum TD Quote size

[Kuppuswamy Sathyanarayanan]

On 6/12/2026 4:08 AM, Peter Fang wrote:
> TDX attestation blob ("TD Quote") sizes can grow with newer
> cryptographic schemes, so guests can no longer rely on a fixed-size
> buffer for the Quote.
> 
> Newer TDX modules report the maximum TD Quote size via a TD-scope
> metadata field. Add a helper to query it instead of exposing tdg_vm_rd()
> directly, as it can read arbitrary metadata fields.
> 
> Thanks to Xu Yilun for suggesting this.

I'm not sure what the original suggestion was. Perhaps a link to the discussion
(or Xu Yilun's email) would be helpful for context.

> 
> Assisted-by: Claude:claude-opus-4-7
> Assisted-by: GitHub Copilot:gpt-5.4
> Signed-off-by: Peter Fang <peter.fang@xxxxxxxxx>
> ---

Looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>



>  arch/x86/coco/tdx/tdx.c           | 19 +++++++++++++++++++
>  arch/x86/include/asm/shared/tdx.h |  1 +
>  arch/x86/include/asm/tdx.h        |  2 ++
>  3 files changed, 22 insertions(+)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 186915a17c50..88c66c46e70a 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -197,6 +197,25 @@ u64 tdx_hcall_get_quote(u8 *buf, size_t size)
>  }
>  EXPORT_SYMBOL_GPL(tdx_hcall_get_quote);
>  
> +/**
> + * tdx_get_max_quote_size() - Get the maximum TD Quote size
> + *
> + * Read the maximum size of a TD Quote from a 4-byte TD metadata field. The TDX
> + * guest driver uses it to size the buffer for Quote retrieval. Older TDX
> + * modules do not support this field and return an error.
> + *
> + * Return: Maximum Quote size in bytes on success, or 0 on failure.
> + */
> +u32 tdx_get_max_quote_size(void)
> +{
> +	u64 val, ret;
> +
> +	ret = tdg_vm_rd(TDCS_QUOTE_MAX_SIZE, &val);
> +
> +	return ret ? 0 : (u32)val;
> +}
> +EXPORT_SYMBOL_GPL(tdx_get_max_quote_size);
> +
>  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 049638e3da74..2880f493a8e5 100644
> --- a/arch/x86/include/asm/shared/tdx.h
> +++ b/arch/x86/include/asm/shared/tdx.h
> @@ -49,6 +49,7 @@
>  /* 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		0x9010000200000008
>  #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 a149740b24e8..ac39674c9479 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -72,6 +72,8 @@ int tdx_mcall_extend_rtmr(u8 index, u8 *data);
>  
>  u64 tdx_hcall_get_quote(u8 *buf, size_t size);
>  
> +u32 tdx_get_max_quote_size(void);
> +
>  void __init tdx_dump_attributes(u64 td_attr);
>  void __init tdx_dump_td_ctls(u64 td_ctls);
>  

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer