linux-kernel - [PATCH MANUALSEL 5.15 8/8] KVM: arm64: Cap KVM_CAP_NR_VCPUS by kvm_arm_default_max

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Date:   Tue, 23 Nov 2021 11:36:30 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sasha Levin <sashal@...nel.org>, catalin.marinas@....com,
        will@...nel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu
Subject: [PATCH MANUALSEL 5.15 8/8] KVM: arm64: Cap KVM_CAP_NR_VCPUS by kvm_arm_default_max_vcpus()

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

[ Upstream commit f60a00d7295057cb4baea5a321501efc72794453 ]

Generally, it doesn't make sense to return the recommended maximum number
of vCPUs which exceeds the maximum possible number of vCPUs.

Note: ARM64 is special as the value returned by KVM_CAP_MAX_VCPUS differs
depending on whether it is a system-wide ioctl or a per-VM one. Previously,
KVM_CAP_NR_VCPUS didn't have this difference and it seems preferable to
keep the status quo. Cap KVM_CAP_NR_VCPUS by kvm_arm_default_max_vcpus()
which is what gets returned by system-wide KVM_CAP_MAX_VCPUS.

Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
Message-Id: <20211116163443.88707-2-vkuznets@...hat.com>
Acked-by: Marc Zyngier <maz@...nel.org>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/arm64/kvm/arm.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 9b328bb05596a..1ed82b6d5e8db 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -223,7 +223,14 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 		r = 1;
 		break;
 	case KVM_CAP_NR_VCPUS:
-		r = num_online_cpus();
+		/*
+		 * ARM64 treats KVM_CAP_NR_CPUS differently from all other
+		 * architectures, as it does not always bound it to
+		 * KVM_CAP_MAX_VCPUS. It should not matter much because
+		 * this is just an advisory value.
+		 */
+		r = min_t(unsigned int, num_online_cpus(),
+			  kvm_arm_default_max_vcpus());
 		break;
 	case KVM_CAP_MAX_VCPUS:
 	case KVM_CAP_MAX_VCPU_ID:
-- 
2.33.0