virt: sev-guest: Remove is_vmpck_empty() helper

Remove is_vmpck_empty() which uses a local array allocation to check if the
VMPCK is empty and replace it with memchr_inv() to directly determine if the
VMPCK is empty without additional memory allocation.

  [ bp: Massage commit message. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20250106124633.1418972-2-nikunj@amd.com

Author: nikunjad

drivers/virt/coco/sev-guest/sev-guest.c

@@ -63,16 +63,6 @@ MODULE_PARM_DESC(vmpck_id, "The VMPCK ID to use when communicating with the PSP.
 /* Mutex to serialize the shared buffer access and command handling. */
 static DEFINE_MUTEX(snp_cmd_mutex);
 
-static bool is_vmpck_empty(struct snp_msg_desc *mdesc)
-{
-	char zero_key[VMPCK_KEY_LEN] = {0};
-
-	if (mdesc->vmpck)
-		return !memcmp(mdesc->vmpck, zero_key, VMPCK_KEY_LEN);
-
-	return true;
-}
-
 /*
  * If an error is received from the host or AMD Secure Processor (ASP) there
  * are two options. Either retry the exact same encrypted request or discontinue
@@ -335,7 +325,7 @@ static int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_r
 	guard(mutex)(&snp_cmd_mutex);
 
 	/* Check if the VMPCK is not empty */
-	if (is_vmpck_empty(mdesc)) {
+	if (!mdesc->vmpck || !memchr_inv(mdesc->vmpck, 0, VMPCK_KEY_LEN)) {
 		pr_err_ratelimited("VMPCK is disabled\n");
 		return -ENOTTY;
 	}
@@ -1024,7 +1014,7 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 	}
 
 	/* Verify that VMPCK is not zero. */
-	if (is_vmpck_empty(mdesc)) {
+	if (!memchr_inv(mdesc->vmpck, 0, VMPCK_KEY_LEN)) {
 		dev_err(dev, "Empty VMPCK%d communication key\n", vmpck_id);
 		goto e_unmap;
 	}