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Thu, 20 Apr 2023 09:16:43 +0100 Date: Thu, 20 Apr 2023 09:16:43 +0100 Message-ID: <86y1mnj7dg.wl-maz@kernel.org> From: Marc Zyngier To: Reiji Watanabe Cc: Oliver Upton , kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, James Morse , Alexandru Elisei , Zenghui Yu , Suzuki K Poulose , Paolo Bonzini , Ricardo Koller , Jing Zhang , Raghavendra Rao Anata , Will Deacon Subject: Re: [PATCH v1 1/2] KVM: arm64: Acquire mp_state_lock in kvm_arch_vcpu_ioctl_vcpu_init() In-Reply-To: <20230420021302.iyl3pqo3lg6lpabv@google.com> References: <20230419021852.2981107-1-reijiw@google.com> <20230419021852.2981107-2-reijiw@google.com> <87cz405or6.wl-maz@kernel.org> <20230420021302.iyl3pqo3lg6lpabv@google.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/28.2 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: reijiw@google.com, oliver.upton@linux.dev, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, james.morse@arm.com, alexandru.elisei@arm.com, yuzenghui@huawei.com, suzuki.poulose@arm.com, pbonzini@redhat.com, ricarkol@google.com, jingzhangos@google.com, rananta@google.com, will@kernel.org X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230420_011647_238311_F7034E9E X-CRM114-Status: GOOD ( 34.63 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, 20 Apr 2023 03:13:02 +0100, Reiji Watanabe wrote: > > Hi Marc, > > On Wed, Apr 19, 2023 at 08:12:45AM +0100, Marc Zyngier wrote: > > On Wed, 19 Apr 2023 03:18:51 +0100, > > Reiji Watanabe wrote: > > > kvm_arch_vcpu_ioctl_vcpu_init() doesn't acquire mp_state_lock > > > when setting the mp_state to KVM_MP_STATE_RUNNABLE. Fix the > > > code to acquire the lock. > > > > > > Signed-off-by: Reiji Watanabe > > > --- > > > arch/arm64/kvm/arm.c | 5 ++++- > > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > > > index fbafcbbcc463..388aa4f18f21 100644 > > > --- a/arch/arm64/kvm/arm.c > > > +++ b/arch/arm64/kvm/arm.c > > > @@ -1244,8 +1244,11 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu, > > > */ > > > if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features)) > > > kvm_arm_vcpu_power_off(vcpu); > > > - else > > > + else { > > > + spin_lock(&vcpu->arch.mp_state_lock); > > > WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_RUNNABLE); > > > + spin_unlock(&vcpu->arch.mp_state_lock); > > > + } > > > > > > return 0; > > > } > > > > I'm not entirely convinced that this fixes anything. What does the > > lock hazard against given that the write is atomic? But maybe a > > It appears that kvm_psci_vcpu_on() expects the vCPU's mp_state > to not be changed by holding the lock. Although I don't think this > code practically causes any real issues now, I am a little concerned > about leaving one instance that updates mpstate without acquiring the > lock, in terms of future maintenance, as holding the lock won't prevent > mp_state from being updated. > > What do you think ? Right, fair enough. It is probably better to take the lock and not have to think of this sort of things... I'm becoming more lazy by the minute! > > > slightly more readable of this would be to expand the critical section > > this way: > > > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > > index 4ec888fdd4f7..bb21d0c25de7 100644 > > --- a/arch/arm64/kvm/arm.c > > +++ b/arch/arm64/kvm/arm.c > > @@ -1246,11 +1246,15 @@ static int kvm_arch_vcpu_ioctl_vcpu_init(struct kvm_vcpu *vcpu, > > /* > > * Handle the "start in power-off" case. > > */ > > + spin_lock(&vcpu->arch.mp_state_lock); > > + > > if (test_bit(KVM_ARM_VCPU_POWER_OFF, vcpu->arch.features)) > > - kvm_arm_vcpu_power_off(vcpu); > > + __kvm_arm_vcpu_power_off(vcpu); > > else > > WRITE_ONCE(vcpu->arch.mp_state.mp_state, KVM_MP_STATE_RUNNABLE); > > > > + spin_unlock(&vcpu->arch.mp_state_lock); > > + > > return 0; > > } > > > > Thoughts? > > Yes, it looks better! Cool. I've applied this change to your patch, applied the series to the lock inversion branch, and remerged the branch in -next. We're getting there! ;-) M. -- Without deviation from the norm, progress is not possible. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel