[tip: x86/tdx] x86/tdx: Add a helper to query maximum Quote size

[tip-bot2 for Peter Fang]

The following commit has been merged into the x86/tdx branch of tip:

Commit-ID:     0dc5ed13653b013ffc0b38a5e1e3f2b2674ffb09
Gitweb:        https://git.kernel.org/tip/0dc5ed13653b013ffc0b38a5e1e3f2b2674ffb09
Author:        Peter Fang <peter.fang@xxxxxxxxx>
AuthorDate:    Wed, 30 Sep 2026 03:30:54 -07:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CommitterDate: Fri, 02 Oct 2026 04:38:53 -07:00

x86/tdx: Add a helper to query maximum Quote size

Newer crypto algorithms can make Quotes larger, so guests can no longer
rely on a fixed-size buffer.

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.

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@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by:
Link: https://patch.msgid.link/20260930103739.2851980-6-peter.fang@xxxxxxxxx
---
 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 92c2438..02c2871 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_FOR_MODULES(tdx_hcall_get_quote, "tdx-guest");
 
+/*
+ * Ask the TDX module what the largest Quote on the host platform 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 f2cfa71..f1c5439 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 97bf6c8..1fcfaed 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -84,6 +84,8 @@ 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);