[RFC PATCH v2 2/2] x86/virt/seamldr: Support multiple sigstruct pages

[Chao Gao <chao.gao@intel.com>]

Each TDX module update image carries a sigstruct, which is passed along
with the module binary to P-SEAMLDR. 'struct seamldr_params' has a single
physical address slot for it, so a sigstruct can be at most one 4KB page.

TDX modules are moving to a post-quantum signature algorithm. Its
signatures are far larger than the ones in use today, so the sigstruct
no longer fits in 4KB.

The P-SEAMLDR ABI was extended for this. It now has four sigstruct
address slots instead of one, for sigstructs of up to 16KB. A new
'version' value of 1 tells P-SEAMLDR that this layout is in use.

Extend 'struct seamldr_params' to match the new ABI. The sigstruct
address list is already populated in a loop over all sigstruct pages, so
raising SEAMLDR_MAX_NR_SIG_PAGES is enough to fill the new slots

Set 'version' to 1 only when the sigstruct spans more than one page. Old
P-SEAMLDRs reject version 1, so this keeps updates with a single-page
sigstruct working on them.

Do not query P-SEAMLDR for version 1 support. Userspace selects the
update image and is responsible for ensuring the module is compatible
with P-SEAMLDR.

AI was used under supervision to review code and workshop logs

Signed-off-by: Chao Gao <chao.gao@intel.com>
---
v2:
 - drop macros for versions to keep the diff minimal
 - explain why the sigstruct filling code needs no change
---
 arch/x86/virt/vmx/tdx/seamldr.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/virt/vmx/tdx/seamldr.c b/arch/x86/virt/vmx/tdx/seamldr.c
index 087bdf10a9a7..9cef6fff2a6c 100644
--- a/arch/x86/virt/vmx/tdx/seamldr.c
+++ b/arch/x86/virt/vmx/tdx/seamldr.c
@@ -36,7 +36,7 @@
 #define SEAMLDR_PARAMS_NR_PAGES		16
 #define SEAMLDR_NR_MODULE_PA_ENTRIES	(496 + 512 * (SEAMLDR_PARAMS_NR_PAGES - 1))
 #define SEAMLDR_MAX_NR_MODULE_PAGES	(SEAMLDR_NR_MODULE_PA_ENTRIES - SEAMLDR_PARAMS_NR_PAGES)
-#define SEAMLDR_MAX_NR_SIG_PAGES	1
+#define SEAMLDR_MAX_NR_SIG_PAGES	4
 
 /*
  * The seamldr_params "scenario" field specifies the operation mode:
@@ -57,7 +57,7 @@ struct seamldr_params {
 	u32	version;
 	u32	scenario;
 	u64	sigstruct_pages_pa_list[SEAMLDR_MAX_NR_SIG_PAGES];
-	u8	reserved[104];
+	u8	reserved[80];
 	u64	module_nr_pages;
 	u64	module_pages_pa_list[SEAMLDR_NR_MODULE_PA_ENTRIES];
 } __packed;
@@ -166,6 +166,13 @@ static void populate_seamldr_params(struct seamldr_params *params,
 				    const u8 *mod, u32 mod_nr_pages)
 {
 	params->version		= 0;
+	/*
+	 * Use version 1 only for multi-page sigstructs, to stay
+	 * compatible with old P-SEAMLDRs that lack version 1 support.
+	 */
+	if (sig_nr_pages > 1)
+		params->version	= 1;
+
 	params->scenario	= SEAMLDR_SCENARIO_UPDATE;
 	params->module_nr_pages	= mod_nr_pages;
 
-- 
2.52.0