linux-kernel - [PATCH v7 7/8] KVM: x86: Allow userspace set maximum VCPU id for VM

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Date:   Fri,  4 Mar 2022 16:07:24 +0800
From:   Zeng Guang <guang.zeng@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kim Phillips <kim.phillips@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Jethro Beekman <jethro@...tanix.com>,
        Kai Huang <kai.huang@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Robert Hu <robert.hu@...el.com>, Gao Chao <chao.gao@...el.com>,
        Zeng Guang <guang.zeng@...el.com>
Subject: [PATCH v7 7/8] KVM: x86: Allow userspace set maximum VCPU id for VM

Introduce new max_vcpu_id in KVM for x86 architecture. Userspace
can assign maximum possible vcpu id for current VM session using
KVM_CAP_MAX_VCPU_ID of KVM_ENABLE_CAP ioctl().

This is done for x86 only because the sole use case is to guide
memory allocation for PID-pointer table, a structure needed to
enable VMX IPI.

By default, max_vcpu_id set as KVM_MAX_VCPU_IDS.

Suggested-by: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
Signed-off-by: Zeng Guang <guang.zeng@...el.com>
---
 arch/x86/include/asm/kvm_host.h |  6 ++++++
 arch/x86/kvm/x86.c              | 11 +++++++++++
 2 files changed, 17 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6dcccb304775..db16aebd946c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1233,6 +1233,12 @@ struct kvm_arch {
 	hpa_t	hv_root_tdp;
 	spinlock_t hv_root_tdp_lock;
 #endif
+	/*
+	 * VM-scope maximum vCPU ID. Used to determine the size of structures
+	 * that increase along with the maximum vCPU ID, in which case, using
+	 * the global KVM_MAX_VCPU_IDS may lead to significant memory waste.
+	 */
+	u32 max_vcpu_id;
 };
 
 struct kvm_vm_stat {
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4f6fe9974cb5..ca17cc452bd3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5994,6 +5994,13 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		kvm->arch.exit_on_emulation_error = cap->args[0];
 		r = 0;
 		break;
+	case KVM_CAP_MAX_VCPU_ID:
+		if (cap->args[0] <= KVM_MAX_VCPU_IDS) {
+			kvm->arch.max_vcpu_id = cap->args[0];
+			r = 0;
+		} else
+			r = -E2BIG;
+		break;
 	default:
 		r = -EINVAL;
 		break;
@@ -11067,6 +11074,9 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 	struct page *page;
 	int r;
 
+	if (vcpu->vcpu_id >= vcpu->kvm->arch.max_vcpu_id)
+		return -E2BIG;
+
 	vcpu->arch.last_vmentry_cpu = -1;
 	vcpu->arch.regs_avail = ~0;
 	vcpu->arch.regs_dirty = ~0;
@@ -11589,6 +11599,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	spin_lock_init(&kvm->arch.hv_root_tdp_lock);
 	kvm->arch.hv_root_tdp = INVALID_PAGE;
 #endif
+	kvm->arch.max_vcpu_id = KVM_MAX_VCPU_IDS;
 
 	INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);
 	INIT_DELAYED_WORK(&kvm->arch.kvmclock_sync_work, kvmclock_sync_fn);
-- 
2.27.0