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SubjectRe: [PATCH] KVM: arm/arm64: replacing per-VM's per-CPU variable
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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 <peng.hao2@zte.com.cn>
> ---
> 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.

Thanks,

M.
--
Jazz is not dead. It just smells funny...

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