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

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

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.

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;
+
+	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);
 
-- 
2.53.0