From mboxrd@z Thu Jan 1 00:00:00 1970 From: Maxim Levitsky Date: Wed, 27 Oct 2021 09:50:30 +0000 Subject: Re: [PATCH v2 02/43] KVM: SVM: Ensure target pCPU is read once when signalling AVIC doorbell Message-Id: List-Id: References: <20211009021236.4122790-1-seanjc@google.com> <20211009021236.4122790-3-seanjc@google.com> In-Reply-To: <20211009021236.4122790-3-seanjc@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Sean Christopherson , Marc Zyngier , Huacai Chen , Aleksandar Markovic , Paul Mackerras , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Paolo Bonzini Cc: James Morse , Alexandru Elisei , Suzuki K Poulose , Atish Patra , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, David Matlack , Oliver Upton , Jing Zhang On Fri, 2021-10-08 at 19:11 -0700, Sean Christopherson wrote: > Ensure vcpu->cpu is read once when signalling the AVIC doorbell. If the > compiler rereads the field and the vCPU is migrated between the check and > writing the doorbell, KVM would signal the wrong physical CPU. Since vcpu->cpu can change any moment anyway, adding READ_ONCE I think can't really fix anything but I do agree that it makes this more readable. Reviewed-by: Maxim Levitsky > > Functionally, signalling the wrong CPU in this case is not an issue as > task migration means the vCPU has exited and will pick up any pending > interrupts on the next VMRUN. Add the READ_ONCE() purely to clean up the > code. > > Opportunistically add a comment explaining the task migration behavior, > and rename cpuid=>cpu to avoid conflating the CPU number with KVM's more > common usage of CPUID. > > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/svm/avic.c | 13 ++++++++++--- > 1 file changed, 10 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c > index 8052d92069e0..208c5c71e827 100644 > --- a/arch/x86/kvm/svm/avic.c > +++ b/arch/x86/kvm/svm/avic.c > @@ -675,10 +675,17 @@ int svm_deliver_avic_intr(struct kvm_vcpu *vcpu, int vec) > smp_mb__after_atomic(); > > if (avic_vcpu_is_running(vcpu)) { > - int cpuid = vcpu->cpu; > + int cpu = READ_ONCE(vcpu->cpu); > > - if (cpuid != get_cpu()) > - wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpuid)); > + /* > + * Note, the vCPU could get migrated to a different pCPU at any > + * point, which could result in signalling the wrong/previous > + * pCPU. But if that happens the vCPU is guaranteed to do a > + * VMRUN (after being migrated) and thus will process pending > + * interrupts, i.e. a doorbell is not needed (and the spurious) > + */ > + if (cpu != get_cpu()) > + wrmsrl(SVM_AVIC_DOORBELL, kvm_cpu_get_apicid(cpu)); > put_cpu(); > } else > kvm_vcpu_wake_up(vcpu);