virt: sev-guest: Carve out SNP message context structure

Currently, the sev-guest driver is the only user of SNP guest messaging.
The snp_guest_dev structure holds all the allocated buffers, secrets page
and VMPCK details. In preparation for adding messaging allocation and
initialization APIs, decouple snp_guest_dev from messaging-related
information by carving out the guest message context
structure(snp_msg_desc).

Incorporate this newly added context into snp_send_guest_request() and all
related functions, replacing the use of the snp_guest_dev.

No functional change.

Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20241009092850.197575-7-nikunj@amd.com

Author: nikunjad

arch/x86/include/asm/sev.h

@@ -234,6 +234,27 @@ struct snp_secrets_page {
 	u8 rsvd4[3744];
 } __packed;
 
+struct snp_msg_desc {
+	/* request and response are in unencrypted memory */
+	struct snp_guest_msg *request, *response;
+
+	/*
+	 * Avoid information leakage by double-buffering shared messages
+	 * in fields that are in regular encrypted memory.
+	 */
+	struct snp_guest_msg secret_request, secret_response;
+
+	struct snp_secrets_page *secrets;
+	struct snp_req_data input;
+
+	void *certs_data;
+
+	struct aesgcm_ctx *ctx;
+
+	u32 *os_area_msg_seqno;
+	u8 *vmpck;
+};
+
 /*
  * The SVSM Calling Area (CA) related structures.
  */