linux-kernel - [PATCH 2/2] kvm: sev: If ccp is busy, report throttled to guest

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Date:   Tue,  8 Nov 2022 21:32:26 +0000
From:   Dionna Glaze <dionnaglaze@...gle.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Dionna Glaze <dionnaglaze@...gle.com>
Subject: [PATCH 2/2] kvm: sev: If ccp is busy, report throttled to guest

The ccp driver can be overloaded even with 1 HZ throttling. The return
value of -EBUSY means that there is no firmware error to report back to
user space, so the guest VM would see this as exitinfo2 = 0. The false
success can trick the guest to update its the message sequence number
when it shouldn't have.

Instead, when ccp returns -EBUSY, we report that to userspace as the
throttling return value.

Signed-off-by: Dionna Glaze <dionnaglaze@...gle.com>
---
 arch/x86/kvm/svm/sev.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index a9f67bfd60d9..6493fb527110 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3641,7 +3641,13 @@ static void snp_handle_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t
 		goto unlock;
 
 	rc = sev_issue_cmd(kvm, SEV_CMD_SNP_GUEST_REQUEST, &data, &err);
-	if (rc)
+	/*
+	 * The ccp driver can return -EBUSY if the PSP is overloaded, so
+	 * we offer that as a throttling signal too.
+	 */
+	if (rc == -EBUSY)
+		rc = SNP_GUEST_REQ_THROTTLED;
+	else if (rc)
 		/* use the firmware error code */
 		rc = err;
 
@@ -3698,7 +3704,14 @@ static void snp_handle_ext_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gp
 
 	rc = snp_guest_ext_guest_request(&req, (unsigned long)sev->snp_certs_data,
 					 &data_npages, &err);
-	if (rc) {
+	/*
+	 * The ccp driver can return -EBUSY if the PSP is overloaded, so
+	 * we offer that as a throttling signal too.
+	 */
+	if (rc == -EBUSY) {
+               rc = SNP_GUEST_REQ_THROTTLED;
+	       goto cleanup;
+	} else if (rc) {
 		/*
 		 * If buffer length is small then return the expected
 		 * length in rbx.
-- 
2.38.1.431.g37b22c650d-goog