linux-kernel - [PATCH MANUALSEL 5.4 2/4] KVM: s390: Cap KVM_CAP_NR_VCPUS by num_online

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Date:   Tue, 23 Nov 2021 11:37:01 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sasha Levin <sashal@...nel.org>, frankja@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org
Subject: [PATCH MANUALSEL 5.4 2/4] KVM: s390: Cap KVM_CAP_NR_VCPUS by num_online_cpus()

From: Vitaly Kuznetsov <vkuznets@...hat.com>

[ Upstream commit 82cc27eff4486f8e79ef8faac1af1f5573039aa4 ]

KVM_CAP_NR_VCPUS is a legacy advisory value which on other architectures
return num_online_cpus() caped by KVM_CAP_NR_VCPUS or something else
(ppc and arm64 are special cases). On s390, KVM_CAP_NR_VCPUS returns
the same as KVM_CAP_MAX_VCPUS and this may turn out to be a bad
'advice'. Switch s390 to returning caped num_online_cpus() too.

Acked-by: Christian Borntraeger <borntraeger@...ibm.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
Reviewed-by: Christian Borntraeger <borntraeger@...ux.ibm.com>
Message-Id: <20211116163443.88707-6-vkuznets@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/s390/kvm/kvm-s390.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index b286818d8d54d..fa11b42683901 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -550,6 +550,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 			r = KVM_MAX_VCPUS;
 		else if (sclp.has_esca && sclp.has_64bscao)
 			r = KVM_S390_ESCA_CPU_SLOTS;
+		if (ext == KVM_CAP_NR_VCPUS)
+			r = min_t(unsigned int, num_online_cpus(), r);
 		break;
 	case KVM_CAP_S390_COW:
 		r = MACHINE_HAS_ESOP;
-- 
2.33.0