From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Jones Subject: Re: [PATCH v2 8/9] KVM: arm/arm64: fix race in kvm_psci_vcpu_on Date: Wed, 5 Apr 2017 10:35:59 +0200 Message-ID: <20170405083559.k3ivjzgevcksjoaq@kamzik.brq.redhat.com> References: <20170331160658.4331-1-drjones@redhat.com> <20170331160658.4331-9-drjones@redhat.com> <20170404194208.GH31208@cbox> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Levente Kurusa , kvm@vger.kernel.org, marc.zyngier@arm.com, pbonzini@redhat.com, kvmarm@lists.cs.columbia.edu To: Christoffer Dall Return-path: Content-Disposition: inline In-Reply-To: <20170404194208.GH31208@cbox> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu List-Id: kvm.vger.kernel.org On Tue, Apr 04, 2017 at 09:42:08PM +0200, Christoffer Dall wrote: > On Fri, Mar 31, 2017 at 06:06:57PM +0200, Andrew Jones wrote: > > From: Levente Kurusa > > > > When two vcpus issue PSCI_CPU_ON on the same core at the same time, > > then it's possible for them to both enter the target vcpu's setup > > at the same time. This results in unexpected behaviors at best, > > and the potential for some nasty bugs at worst. > > > > Signed-off-by: Levente Kurusa > > Signed-off-by: Andrew Jones > > --- > > arch/arm/kvm/psci.c | 4 ++-- > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > diff --git a/arch/arm/kvm/psci.c b/arch/arm/kvm/psci.c > > index f732484abc7a..0204daa899b1 100644 > > --- a/arch/arm/kvm/psci.c > > +++ b/arch/arm/kvm/psci.c > > @@ -88,7 +88,8 @@ static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu) > > */ > > if (!vcpu) > > return PSCI_RET_INVALID_PARAMS; > > - if (!test_bit(KVM_REQ_POWER_OFF, &vcpu->requests)) { > > + > > + if (!test_and_clear_bit(KVM_REQ_POWER_OFF, &vcpu->requests)) { > > if (kvm_psci_version(source_vcpu) != KVM_ARM_PSCI_0_1) > > return PSCI_RET_ALREADY_ON; > > else > > @@ -116,7 +117,6 @@ static unsigned long kvm_psci_vcpu_on(struct kvm_vcpu *source_vcpu) > > * the general puspose registers are undefined upon CPU_ON. > > */ > > vcpu_set_reg(vcpu, 0, context_id); > > - clear_bit(KVM_REQ_POWER_OFF, &vcpu->requests); > > > > wq = kvm_arch_vcpu_wq(vcpu); > > swake_up(wq); > > -- > > 2.9.3 > > > > Depending on what you end up doing with the requests, if you keep the > bool flag you could just use the kvm->lock mutex instead. > > Have you considered if there are any potential races between > kvm_psci_system_off() being called on one VCPU while two other VPCUs are > turning on the same CPU that is being turend off as part of system-wide > power down as well? Sounds like a nice unit test. I haven't considered it, but I guess the kvm_psci_system_off/reset calling VCPU will ultimately "win", as it'll cause an exit to userspace that initiates a shutdown/reset. When the VCPUs are restarted then vcpu init should reset the power_off state correctly. As long as the race this patch addresses is fixed, then I'm not sure there should be any risk with the actual system_off/reset being delayed wrt a vcpu being "on'ed" again, nor with there being more than one VCPU trying to "on" it at the same time. > > I'm wondering if this means we should take the kvm->lock at a higher > level when handling PSCI events... That would simplify our analysis of the PSCI emulation, but I'm not sure we want to give a guest the power to constantly acquire that mutex with a barrage of PSCI calls. Maybe we should create a PSCI mutex? In order to avoid holding it too long we may want power_off to be more than a boolean though, i.e. the PENDING state might also be a good idea to represent. Thanks, drew