Re: [PATCH v15 01/13] x86/sev: Carve out and export SNP guest messaging init routines

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 12/3/2024 7:49 PM, Borislav Petkov wrote:
> On Tue, Dec 03, 2024 at 02:30:33PM +0530, Nikunj A Dadhania wrote:
>> @@ -458,6 +456,20 @@ void set_pte_enc_mask(pte_t *kpte, unsigned long pfn, pgprot_t new_prot);
>>  void snp_kexec_finish(void);
>>  void snp_kexec_begin(void);
>>  
>> +static inline bool snp_is_vmpck_empty(struct snp_msg_desc *mdesc)
>> +{
>> +	static const char zero_key[VMPCK_KEY_LEN] = {0};
>> +
>> +	if (mdesc->vmpck)
>> +		return !memcmp(mdesc->vmpck, zero_key, VMPCK_KEY_LEN);
>> +
>> +	return true;
>> +}
> 
> This function looks silly in a header with that array allocation.
> 
> I think you should simply do:
> 
> 	if (memchr_inv(mdesc->vmpck, 0, VMPCK_KEY_LEN))

Just a minor nit, it will need a negation:

 	if (!memchr_inv(mdesc->vmpck, 0, VMPCK_KEY_LEN))
 
> at the call sites and not have this helper at all.
> 
> But please do verify whether what I'm saying actually makes sense and if it
> does, this can be a cleanup pre-patch.

Yes, it makes sense. I have verified the code and below is the cleanup pre-patch.

From 4d249f393aeba7bed7fb99778b8ee8a24a33b5b7 Mon Sep 17 00:00:00 2001
From: Nikunj A Dadhania <nikunj@amd.com>
Date: Tue, 3 Dec 2024 20:48:28 +0530
Subject: [PATCH] virt: sev-guest: Remove is_vmpck_empty() helper

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

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
---
 drivers/virt/coco/sev-guest/sev-guest.c | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index b699771be029..62328d0b2cb6 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/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;
 	}
-- 
2.34.1