linux-kernel - [PATCH v2 2/2] kvm: sev: If ccp is busy, report throttled to guest

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Message-ID: <20221117181127.1859634-3-dionnaglaze@google.com>
Date:   Thu, 17 Nov 2022 18:11:27 +0000
From:   Dionna Glaze <dionnaglaze@...gle.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Dionna Glaze <dionnaglaze@...gle.com>,
        Thomas Lendacky <Thomas.Lendacky@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joerg Roedel <jroedel@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirsky <luto@...nel.org>,
        John Allen <john.allen@....com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Peter Gonda <pgonda@...gle.com>
Subject: [PATCH v2 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.

Cc: Thomas Lendacky <Thomas.Lendacky@....com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Joerg Roedel <jroedel@...e.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Andy Lutomirsky <luto@...nel.org>
Cc: John Allen <john.allen@....com>
Cc: Herbert Xu <herbert@...dor.apana.org.au>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Peter Gonda <pgonda@...gle.com>

Signed-off-by: Dionna Glaze <dionnaglaze@...gle.com>
---
 arch/x86/kvm/svm/sev.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 3ced06c6e07a..81e4862126fb 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3335,7 +3335,13 @@ 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;
 
@@ -3368,7 +3374,7 @@ static void snp_handle_ext_guest_request(struct vcpu_svm *svm, gpa_t req_gpa, gp
 	if (snp_throttle_guest_request(sev)) {
 		rc = SNP_GUEST_REQ_THROTTLED;
 		goto e_fail;
-	}
+       }
 
 	data_gpa = vcpu->arch.regs[VCPU_REGS_RAX];
 	data_npages = vcpu->arch.regs[VCPU_REGS_RBX];
@@ -3392,7 +3398,14 @@ 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.
-- 
2.38.1.584.g0f3c55d4c2-goog