From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752605AbeCMTeA (ORCPT ); Tue, 13 Mar 2018 15:34:00 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:43596 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751534AbeCMTd6 (ORCPT ); Tue, 13 Mar 2018 15:33:58 -0400 Subject: Re: [PATCH] KVM: arm/arm64: replacing per-VM's per-CPU variable To: Marc Zyngier , Peng Hao , linux@armlinux.org.uk, catalin.marinas@arm.com, will.deacon@arm.com, cdall@kernel.org Cc: kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <1520974993-40423-1-git-send-email-peng.hao2@zte.com.cn> <37d19d6c-8787-d190-0ef0-c6709cb9e976@arm.com> From: Robin Murphy Message-ID: <01c88eaa-e863-1103-8bb1-b723b4d3ab87@arm.com> Date: Tue, 13 Mar 2018 19:33:55 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <37d19d6c-8787-d190-0ef0-c6709cb9e976@arm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 13/03/18 13:01, Marc Zyngier wrote: > [You're repeatedly posting to the kvmarm mailing list without being > subscribed to it. I've flushed the queue now, but please consider > subscribing to the list, it will help everyone] > > On 13/03/18 21:03, Peng Hao wrote: >> Using a global per-CPU variable instead of per-VM's per-CPU variable. >> >> Signed-off-by: Peng Hao >> --- >> arch/arm/include/asm/kvm_host.h | 3 --- >> arch/arm64/include/asm/kvm_host.h | 3 --- >> virt/kvm/arm/arm.c | 26 ++++++-------------------- >> 3 files changed, 6 insertions(+), 26 deletions(-) >> >> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h >> index 248b930..4224f3b 100644 >> --- a/arch/arm/include/asm/kvm_host.h >> +++ b/arch/arm/include/asm/kvm_host.h >> @@ -59,9 +59,6 @@ struct kvm_arch { >> /* VTTBR value associated with below pgd and vmid */ >> u64 vttbr; >> >> - /* The last vcpu id that ran on each physical CPU */ >> - int __percpu *last_vcpu_ran; >> - >> /* >> * Anything that is not used directly from assembly code goes >> * here. >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h >> index 596f8e4..5035a08 100644 >> --- a/arch/arm64/include/asm/kvm_host.h >> +++ b/arch/arm64/include/asm/kvm_host.h >> @@ -67,9 +67,6 @@ struct kvm_arch { >> /* VTTBR value associated with above pgd and vmid */ >> u64 vttbr; >> >> - /* The last vcpu id that ran on each physical CPU */ >> - int __percpu *last_vcpu_ran; >> - >> /* The maximum number of vCPUs depends on the used GIC model */ >> int max_vcpus; >> >> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c >> index 86941f6..a67ffb0 100644 >> --- a/virt/kvm/arm/arm.c >> +++ b/virt/kvm/arm/arm.c >> @@ -59,6 +59,8 @@ >> /* Per-CPU variable containing the currently running vcpu. */ >> static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_arm_running_vcpu); >> >> +static DEFINE_PER_CPU(struct kvm_vcpu *, kvm_last_ran_vcpu); >> + >> /* The VMID used in the VTTBR */ >> static atomic64_t kvm_vmid_gen = ATOMIC64_INIT(1); >> static u32 kvm_next_vmid; >> @@ -115,18 +117,11 @@ void kvm_arch_check_processor_compat(void *rtn) >> */ >> int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) >> { >> - int ret, cpu; >> + int ret; >> >> if (type) >> return -EINVAL; >> >> - kvm->arch.last_vcpu_ran = alloc_percpu(typeof(*kvm->arch.last_vcpu_ran)); >> - if (!kvm->arch.last_vcpu_ran) >> - return -ENOMEM; >> - >> - for_each_possible_cpu(cpu) >> - *per_cpu_ptr(kvm->arch.last_vcpu_ran, cpu) = -1; >> - >> ret = kvm_alloc_stage2_pgd(kvm); >> if (ret) >> goto out_fail_alloc; >> @@ -147,9 +142,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) >> return ret; >> out_free_stage2_pgd: >> kvm_free_stage2_pgd(kvm); >> -out_fail_alloc: >> - free_percpu(kvm->arch.last_vcpu_ran); >> - kvm->arch.last_vcpu_ran = NULL; >> +out_fail_alloc: >> return ret; >> } >> >> @@ -179,9 +172,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm) >> >> kvm_vgic_destroy(kvm); >> >> - free_percpu(kvm->arch.last_vcpu_ran); >> - kvm->arch.last_vcpu_ran = NULL; >> - >> for (i = 0; i < KVM_MAX_VCPUS; ++i) { >> if (kvm->vcpus[i]) { >> kvm_arch_vcpu_free(kvm->vcpus[i]); >> @@ -343,17 +333,13 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu) >> >> void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu) >> { >> - int *last_ran; >> - >> - last_ran = this_cpu_ptr(vcpu->kvm->arch.last_vcpu_ran); >> - >> /* >> * We might get preempted before the vCPU actually runs, but >> * over-invalidation doesn't affect correctness. >> */ >> - if (*last_ran != vcpu->vcpu_id) { >> + if (per_cpu(kvm_last_ran_vcpu, cpu) != vcpu) { >> kvm_call_hyp(__kvm_tlb_flush_local_vmid, vcpu); >> - *last_ran = vcpu->vcpu_id; >> + per_cpu(kvm_last_ran_vcpu, cpu) = vcpu; >> } >> >> vcpu->cpu = cpu; >> > > Have you read and understood what this code is about? The whole point of > this code is to track physical CPUs on a per-VM basis. Making it global > completely defeats the point, and can result in guest memory corruption. > Please see commit 94d0e5980d67. I won't comment on the patch itself (AFAICS it is rather broken), but I suppose there is a grain of sense in the general idea, since the set of physical CPUs itself is fundamentally a global thing. Given a large number of pCPUs and a large number of VMs it could well be more space-efficient to keep a single per-pCPU record of a {vmid,vcpu_id} tuple or some other *globally-unique* vCPU namespace (I guess just the struct kvm_vcpu pointer might work, but then it would be hard to avoid unnecessary invalidation when switching VMIDs entirely). Robin.