linux-kernel - [PATCH v3 1/4] KVM: Add per vCPU flag specifying that a vCPU is loaded

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Date:   Mon,  2 Oct 2023 14:57:20 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Sean Christopherson <seanjc@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH v3 1/4] KVM: Add per vCPU flag specifying that a vCPU is loaded

Add vcpu->loaded boolean flag specifying that a vCPU is loaded.

Such flag can be useful in a vendor code (e.g AVIC) to make
decisions based on it.

Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 include/linux/kvm_host.h |  1 +
 virt/kvm/kvm_main.c      | 10 ++++++++++
 2 files changed, 11 insertions(+)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index fb6c6109fdcad69..331432d86e44d51 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -379,6 +379,7 @@ struct kvm_vcpu {
 #endif
 	bool preempted;
 	bool ready;
+	bool loaded;
 	struct kvm_vcpu_arch arch;
 	struct kvm_vcpu_stat stat;
 	char stats_id[KVM_STATS_NAME_SIZE];
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 486800a7024b373..615f2a02b7cb97f 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -214,6 +214,10 @@ void vcpu_load(struct kvm_vcpu *vcpu)
 	__this_cpu_write(kvm_running_vcpu, vcpu);
 	preempt_notifier_register(&vcpu->preempt_notifier);
 	kvm_arch_vcpu_load(vcpu, cpu);
+
+	/* Ensure that vcpu->cpu is visible before vcpu->loaded is set to true */
+	smp_wmb();
+	WRITE_ONCE(vcpu->loaded, true);
 	put_cpu();
 }
 EXPORT_SYMBOL_GPL(vcpu_load);
@@ -221,6 +225,12 @@ EXPORT_SYMBOL_GPL(vcpu_load);
 void vcpu_put(struct kvm_vcpu *vcpu)
 {
 	preempt_disable();
+	WRITE_ONCE(vcpu->loaded, false);
+	/*
+	 * Ensure that vcpu->loaded is set and visible,
+	 * before KVM actually unloads the vCPU.
+	 */
+	smp_wmb();
 	kvm_arch_vcpu_put(vcpu);
 	preempt_notifier_unregister(&vcpu->preempt_notifier);
 	__this_cpu_write(kvm_running_vcpu, NULL);
-- 
2.26.3