linux-kernel - [PATCH 1/4] KVM: SVM: Don't set current

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Date:   Tue,  6 Apr 2021 10:18:08 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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,
        linux-kernel@...r.kernel.org, Cathy Avery <cavery@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH 1/4] KVM: SVM: Don't set current_vmcb->cpu when switching vmcb

Do not update the new vmcb's last-run cpu when switching to a different
vmcb.  If the vCPU is migrated between its last run and a vmcb switch,
e.g. for nested VM-Exit, then setting the cpu without marking the vmcb
dirty will lead to KVM running the vCPU on a different physical cpu with
stale clean bit settings.

                          vcpu->cpu    current_vmcb->cpu    hardware
  pre_svm_run()           cpu0         cpu0                 cpu0,clean
  kvm_arch_vcpu_load()    cpu1         cpu0                 cpu0,clean
  svm_switch_vmcb()       cpu1         cpu1                 cpu0,clean
  pre_svm_run()           cpu1         cpu1                 kaboom

Simply delete the offending code; unlike VMX, which needs to update the
cpu at switch time due to the need to do VMPTRLD, SVM only cares about
which cpu last ran the vCPU.

Fixes: af18fa775d07 ("KVM: nSVM: Track the physical cpu of the vmcb vmrun through the vmcb")
Cc: Cathy Avery <cavery@...hat.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 48b396f33bee..89619cc52cf4 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1311,14 +1311,6 @@ void svm_switch_vmcb(struct vcpu_svm *svm, struct kvm_vmcb_info *target_vmcb)
 	svm->current_vmcb = target_vmcb;
 	svm->vmcb = target_vmcb->ptr;
 	svm->vmcb_pa = target_vmcb->pa;
-
-	/*
-	* Track the physical CPU the target_vmcb is running on
-	* in order to mark the VMCB dirty if the cpu changes at
-	* its next vmrun.
-	*/
-
-	svm->current_vmcb->cpu = svm->vcpu.cpu;
 }
 
 static int svm_create_vcpu(struct kvm_vcpu *vcpu)
-- 
2.31.0.208.g409f899ff0-goog